How to Bold, Italicize, or Underline Text in a PDF

To make text bold in a PDF, open the file in an online editor, double-click the text so the cursor appears, drag to highlight the words you want, then click the Bold (B) button in the toolbar. Use the Italic (I) and Underline (U) buttons the same way. Save and download so the formatting is written into the file permanently. Most edits take seconds.

Key takeaways

• Bold, italic, and underline are toolbar toggles once you are inside a text box; select the characters first, then click B, I, or U, exactly like a word processor.
• You must enter edit mode by double-clicking the text, not just clicking the page, or the formatting buttons do nothing.
• The big trap is the embedded font: if the PDF only carries the regular weight of a font, asking for bold can produce a faux-bold smear or fall back to a different typeface entirely.
• Save and download after formatting so the change is baked into the document and survives reopening, emailing, and printing.
• Underline is for emphasis, not deletion; if you want a line through text to mark it as removed, that is a separate strikethrough job.
• No software to install for the basic edit; a browser-based editor handles it on Windows, Mac, iPhone, and Android.

What "bold in a PDF" actually means

In a word processor, bold is a live attribute you can flip on any character at any time, because the program has the full font family on hand. A PDF is different. By the time a document is exported to PDF, each piece of text is locked to a specific font, size, color, and weight, and the file often embeds only the exact glyphs it used. That is why a finished PDF feels frozen, and why editing its formatting is a slightly different exercise than typing in Word.

A real PDF editor gets around this by making the text box live again: it reads the characters, lets you put a cursor in them, and re-renders the run when you change a style. So when you click Bold, the editor is not faking a viewer overlay; it is rewriting that text to a bold weight and saving it back into the page. The same goes for italic and underline. The catch, which we will come back to, is that bold and italic depend on the font actually having those styles available.

This is the same machinery you use to change the font in a PDF: you are editing the text run itself, not annotating over the top of it.

How do I make text bold in a PDF?

Here is the straightforward path in an online editor. You open the file, get inside the text, select the words, and toggle bold.

1. Open your PDF in the editor. Go to the PDF editor and upload the file. It opens in your browser with every page rendered and ready to edit.
2. Double-click the text you want to change. A single click selects the text box as an object; a double-click drops a blinking cursor inside it so you can edit the characters. This step is the one people skip.
3. Highlight the exact words. Drag across the words, or double-click a single word, so they are selected. Whatever is highlighted is what gets the new style; the rest of the line stays as it is.
4. Click Bold (B) in the toolbar. The selected text thickens immediately. Clicking B again toggles it back off, so it works as a switch.
5. Repeat for other passages. Move to the next heading or phrase, select, and toggle. You can mix styles freely within one paragraph.
6. Save and download. Export the file. The bold weight is now part of the document and travels with it to every reader, printer, and inbox.

Italic and underline work identically: highlight the text, then click the Italic (I) button or the Underline (U) button. Many editors also map the familiar keyboard shortcuts once your cursor is in the text, so Ctrl+B (Cmd+B on Mac), Ctrl+I, and Ctrl+U toggle the three styles without reaching for the toolbar.

The catch: bold and italic depend on the embedded font

This is the part nobody warns you about, and it explains almost every "my bold looks wrong" complaint.

When a PDF is created, it usually embeds only the font styles it actually used. If the original document was set entirely in regular-weight Calibri, the file may carry only Calibri Regular. There is no bold Calibri inside it. So when you ask the editor to bold a word, one of three things happens, and the result depends on the tool:

Faux bold and faux oblique are the editor doing its best: it thickens or slants the existing glyph to imitate the missing style. On screen it can look passable, but up close the strokes are uneven and the spacing is slightly off. The cleaner fix, when it matters, is to switch that text to a font whose bold and italic weights are fully available, which is exactly the workflow in our font-changing guide. A good editor handles this for you by substituting a complete family, but it is worth knowing why a bolded heading sometimes looks subtly different from the body text around it.

Underline is the exception here. It is a drawn line under the text, not a font style, so it never depends on the embedded family. Underline always renders cleanly regardless of which font the text uses.

Formatting headings, emphasis, and links the right way

Once you are comfortable toggling styles, a few conventions keep the result looking professional rather than busy.

1. Use bold for headings and key terms. Bolding a section title or a single critical phrase guides the eye. Bolding whole paragraphs defeats the purpose, because when everything is bold, nothing is.
2. Use italic for titles, asides, and foreign words. Book and publication titles, the occasional emphasized word, and Latin or foreign-language terms are the classic italic cases. Italic for long stretches gets tiring to read.
3. Reserve underline for links, or skip it. On screen, underlined text reads as a hyperlink, so underlining a plain phrase can confuse readers into clicking. If you only want emphasis, bold or italic is usually the better signal.
4. Match the size and color while you are in there. If a heading should also be larger or a different color, set those in the same selection so the whole style change happens in one pass.

A practical example: you have a contract draft and want the clause numbers bold, one defined term in italic, and nothing underlined. You double-click into each paragraph, select the clause number and click B, select the defined term and click I, and leave everything else alone. The body stays regular, the structure is suddenly readable, and the file looks intentional rather than marked-up.

Underline versus strikethrough: don't mix them up

These two get conflated constantly, so it is worth a clear line.

Underline draws a line beneath text and signals emphasis or a link. Strikethrough draws a line through text and signals that the words are deleted, superseded, or no longer apply, which is why it is the standard tool for redlining edits and showing tracked changes. They look related but mean opposite things, and using the wrong one sends the wrong message.

If your real goal is to mark text as removed rather than emphasized, the underline button is not what you want; see our dedicated guide on how to strikethrough text in a PDF. Reach for underline only when you mean "look here" or "this is a link," and for the body of any document, be sparing with all three styles so the ones you do use actually stand out.

The scanned-PDF problem

There is one type of PDF where none of this works directly, and it surprises people: a scanned document.

A scan is a picture of a page. The "text" you see is pixels in an image, not editable characters, so when you double-click it nothing happens and there is no cursor to place. You cannot bold an image of a word. Before you can format anything, the text has to be made real, which means running optical character recognition (OCR) to convert the picture into a selectable text layer. Many editors do this on import for scanned files; once the text is recognized, you can select and format it like any other. If your PDF refuses to let you click into the text, a scan with no text layer is the most likely reason, and OCR is the missing step.

Platform variations

You can format PDF text on any device, but the built-in tools vary widely in what they let you actually do.

The honest summary: free built-in viewers on phones and Windows generally let you annotate over a PDF but not restyle the text that is already in it. Mac Preview can underline as a markup but cannot make existing body text bold. For genuine bold, italic, and underline applied to the document's own text, you need either paid desktop software or a browser-based editor. The online route behaves identically across every operating system, which helps when you are moving between a laptop and a phone.

Making the change permanent

Whatever tool you use, formatting only counts once it is saved into the file. Some viewers let you toggle a style on screen or add a temporary markup that is not flattened into the document; reopen the file elsewhere and the emphasis is gone or sits in a separate annotation layer a recipient can remove.

In a real editor this is handled for you: when you download, the bold, italic, and underline are written into the page content, so the file looks the same for everyone who opens it. The rule of thumb is simple. After you format, export and download a fresh copy, then reopen it to confirm the styles held. One quick note on privacy, since you are uploading a document: an online editor processes your file on a server to perform the edit, and files are not kept long-term. That is normal for browser-based editing, but worth knowing before you upload anything sensitive.

FAQ

How do I make text bold in a PDF?

Open the PDF in an online editor, double-click the text so a cursor appears inside it, then drag to highlight the words you want. Click the Bold (B) button in the toolbar, or press Ctrl+B (Cmd+B on Mac). The selected text thickens immediately. Save and download the file so the bold weight is written into the document and stays bold when you reopen, email, or print it.

Why does bold look fuzzy or different after I apply it?

Because the PDF likely embeds only the regular weight of that font, with no true bold available. The editor then fakes it by thickening the existing glyph, which can look uneven up close, or it substitutes a different typeface. The clean fix is to switch that text to a font whose bold weight is fully present, which our font-changing guide walks through. Underline never has this problem since it is a drawn line.

Can I underline text in a PDF without underlining the whole line?

Yes. Double-click into the text to get a cursor, then drag to select only the specific words you want underlined and click the Underline (U) button. Just that selection gets the line; the rest of the line stays plain. Keep in mind that underlined text reads as a hyperlink to many people, so use it deliberately or choose bold or italic for plain emphasis instead.

Why can't I click into the text to format it?

The most common reason is a scanned PDF. A scan is an image of a page, so the words are pixels, not editable characters, and there is no cursor to place. You first need optical character recognition (OCR) to turn the picture into a real text layer; many editors do this on import. The other possibility is that you single-clicked and selected the text box as an object rather than double-clicking to edit inside it.

Does formatting text change the rest of my PDF?

No. Bolding, italicizing, or underlining a selection only affects the characters you highlighted; the surrounding text, images, and page layout stay exactly as they were. The change is local to that text run. Quality elsewhere in the file is untouched, since you are editing one piece of text rather than re-rendering or recompressing the whole document. Always save a fresh copy and reopen it to confirm the styles held.