How to Annotate and Mark Up a PDF Online

A landlord sends you a lease as a PDF and asks you to circle the clauses you have questions about. Or your professor emails back a reading with “see attached” and you need to underline the passages you’ll bring up in class. In both cases you’re staring at a document you can view but can’t mark up — because most PDF viewers, especially the built-in browser one, let you read a page but not draw on it.

The old workaround was printing the page, marking it by hand, and scanning it back in — which is slow, wastes paper, and produces a blurry scan of your own handwriting layered on top of a document that was already digital. A proper PDF annotator skips all of that.

What annotating a PDF actually involves

Annotation means adding pen strokes, highlights, or marks on top of a rendered PDF page without altering the underlying document content — then saving those marks back into a downloadable file. Done right, the result is a normal PDF anyone can open, with your highlights and notes baked in as part of the page image or vector layer.

The key distinction from editing a PDF’s text is that you’re not changing what the document says — you’re marking it up the way you would with a highlighter and pen on paper.

Why people get stuck here

• No native drawing tool. Chrome, Safari, and most free PDF readers let you view and sometimes fill form fields, but not draw freehand or highlight arbitrary text.
• Desktop software overkill. Full PDF editors with annotation features often sit behind a subscription, which is a lot to pay for occasionally circling a paragraph.
• Flattening confusion. Some annotation tools save marks as a separate, removable layer; others “flatten” them permanently into the page. People often don’t realize which one they’re using until the annotations disappear or can’t be edited later.
• Multi-page documents. A contract or reading assignment might be 10-30 pages, and scrolling through a print-and-scan workaround page by page is tedious.

What a good PDF annotator looks like

Real pen and highlighter tools

You should be able to draw freehand and highlight text-like regions directly on the rendered page, not just drop a fixed comment box.

Page-by-page navigation

For a multi-page document, you need to move between pages while keeping your annotations from earlier pages intact, not restart the process for each page separately.

A flattened, shareable output

Once you’re done, the tool should generate a standard downloadable PDF with your marks baked into the pages — something you can email or upload anywhere without the recipient needing special software to see your annotations.

Common mistakes to avoid

• Printing and rescanning a document that was already a clean digital PDF, which introduces blur and file bloat for no reason.
• Marking up a document with a screenshot tool and pasting it into a new file, losing the original page layout and text quality.
• Forgetting to check that your annotations actually survived the save/export step before sending the file on.
• Annotating a scanned document without checking first whether the text underneath is even selectable — see also a searchable-PDF checker if you’re unsure.

How to do it with PDF Annotator

Online Tool Store’s PDF Annotator runs entirely in your browser — the file is processed locally and never uploaded to a server.

1. Open the PDF Annotator tool and load your PDF.
2. Pick the pen or highlighter tool and mark up the page directly — circle a clause, underline a sentence, jot a note in the margin.
3. Move to the next page and repeat; your earlier annotations stay in place.
4. Save the flattened, downloadable PDF once you’re done marking it up.

Because it runs locally, it’s a reasonable choice even for something like a signed lease or a medical form you’d rather not upload to an unfamiliar server.

Frequently asked questions

Can I remove an annotation after I’ve added it?

Within the same editing session, yes — most annotators let you undo or erase a mark before you export. Once the file is flattened and downloaded, though, the marks become part of the page and can’t be selectively removed without annotating over them again.

Will my annotations show up if I open the PDF on another device?

Yes, as long as the tool flattens the marks into the actual page content rather than storing them as a separate proprietary layer, the output is a standard PDF that displays the same everywhere.

Is this the same as filling out a PDF form?

No. Form filling puts text into predefined fields the document already has. Annotation is freehand drawing or highlighting on top of the page itself, which works even on documents that have no form fields at all.

Final thought

If all you need to do is mark up a document — not rewrite its text — skip the print-and-scan loop. A browser-based annotator gets you the same circled clause or highlighted paragraph without the blur, the paper, or the wait.

Try the free PDF Annotator tool