How to Annotate PDFs on Android With a Stylus or S-Pen

A stylus turns a tablet into something genuinely better than paper for marking up documents, but Android's PDF handling varies enough between devices and apps that the default route is often the worst one. The main thing to get right is whether your marks are being saved into the file or just into an app.

Check what the built-in viewer actually does

Opening a PDF from Drive or Files on many Android devices gives you a viewer that renders the document and little else. Some offer basic markup; many do not, and where markup exists the save behaviour is inconsistent — marks may end up in a copy, in app-local storage, or nowhere.

The test takes fifteen seconds and is worth running before you annotate anything important: make one mark, save, close the app completely, reopen the file from its original location, and see whether the mark is there. If it is not, you need a different app.

Pick an app that writes real annotations

What you want is an app that stores your marks as PDF annotations inside the file. Then any reader on any platform sees them, and they travel with the document when you send it.

The alternative — an app that keeps notes in its own database and overlays them at display time — looks identical while you use it and produces a file with nothing in it when you share it. This catches people out constantly.

Dedicated PDF apps on Android generally do this properly and support pressure-sensitive input. Samsung Notes, on Galaxy devices with an S-Pen, handles PDF markup well and is the path of least resistance if you already have it — but check how it exports, because "share as PDF" and "export" can behave differently.

Ink versus text markup

Two different things, useful for different jobs:

Ink is freehand drawing — circles, arrows, marginal notes, handwriting. It does not know what is under it. Perfect for review and for study notes, useless for anything that needs to be searched or listed later.

Text markup — highlight, underline, strikethrough — anchors to the actual characters. It can be listed, exported as a set of quotations, and searched. This is what you want for research.

Text markup needs a real text layer. On a scanned document there is nothing to anchor to, so highlighting silently degrades into a drawn rectangle. If you are marking up scans and need quotations back out later, OCR the file first.

Getting the best out of an S-Pen

• Turn on palm rejection if the app offers it. Without it, resting your hand puts marks on the page.
• Use pressure sensitivity for genuine handwriting; turn it off for consistent-width annotation lines.
• Learn the button. On S-Pen models the side button usually toggles erase or select, which is much faster than going back to the toolbar.
• Zoom in to write. Handwriting at 100% is cramped; zooming and writing larger reads far better when zoomed back out.
• Keep a colour convention. One colour for questions, one for corrections, one for approvals. It turns a marked-up document into something you can act on systematically.

Sending it back, and the trap in doing so

When you share the annotated file, use the app's export or save-as-PDF function rather than a generic share of the open document, and check what arrives. Send it to yourself first and open it in a different reader. If the marks are missing, they were never in the file.

Also decide whether you want your marks to be removable. Annotations are objects a recipient can delete or hide. Flattening burns them permanently into the page — right for a final marked-up record, wrong if the recipient needs to work with a clean document underneath.

When the document itself needs changing

Annotating says "this is wrong". Editing fixes it. If you need to correct the text rather than mark it, this site's editor runs in a mobile browser, edits text in place with the layout preserved, and handles files up to 10 MB and 100 pages.

Frequently asked questions

Why did my annotations disappear? Almost always an app that stored them outside the file, or a save that wrote a copy. Test with the close-and-reopen routine above.

Can I highlight text on a scanned PDF? Not as real text highlights — there is nothing to anchor to. You can draw over it, or OCR it first.

Do I need an S-Pen specifically? No. Any capacitive stylus works for ink. Pressure sensitivity and the side button need a supported active stylus.

Will an iPhone or desktop user see my marks? Yes, if they were saved as real PDF annotations. That is the whole reason to use an app that writes them properly.

How do I stop someone editing my annotations? Flatten the file before sending. It makes them permanent and no longer separately selectable.