How do I highlight a PDF online, even a scan?
Drag a box over the part you want to mark, then download the copy. The highlight is drawn into the page, so it looks the same in every reader and works on scans. New highlights are yellow at 35% opacity. The person you send it to cannot delete them, and the words underneath can still be copied.
Can I highlight a scanned PDF?
Yes. Highlights follow the area you drag, not recognised text, so scanned pages and photos work. Reader highlight tools often fail there, because they find no text to select. The scanned words still will not become searchable.
Can the person I send it to remove my highlights?
No. Each highlight is drawn into the page, not added as a comment, so there is nothing for a reader to select and delete. Keep an unmarked copy. When marks must be removable, or show who made them, use the highlight tool in Acrobat Reader, Preview or your browser.
Two kinds of PDF highlight
The PDF format has a dedicated highlight annotation: a markup object that records the marked text area, an author and an optional comment, and that readers list in a comments panel. The PDFCheck highlighter does not create those. It draws a semi-transparent filled rectangle into the page's own content, the same way a line or a coloured box is drawn. The table shows what that difference means once the file leaves PDFCheck.
| Behaviour | Highlight annotation | PDFCheck drawn highlight |
|---|---|---|
| Listed in a reader's comments panel | Yes | No |
| Can be deleted by a recipient as one item | Yes | No, it is part of the page |
| Shape | Follows the lines of selected text | The exact rectangle you drag |
| Works on scanned pages and pictures | Only where the reader finds text to select | Yes |
| Shown and printed identically in every reader | Depends on reader and print settings | Yes |
| Carries an author name and timestamp | Usually | No |
Measured on a sample file
One highlight was added across the first line of a two-page sample, then the file was saved. The annotation count was identical before and after: two links, one comment, one attachment and one form widget. No new object appeared, confirming the highlight lives in the page content. The words under the yellow area were still extractable as text.
The save also changed the file's properties. The Producer became pdf-lib (https://github.com/Hopding/pdf-lib), the modification date moved to the time of the save, and the XMP metadata stream kept the old producer name. The file was rewritten with compressed object streams and went from 11,908 to 9,465 bytes.
Default colour, size and legibility
- New highlights are
#ffef00yellow at 35% opacity. The text underneath stays dark enough to read on white paper; dark highlight colours or higher opacity reduce contrast quickly. - A drag narrower than 5 screen pixels is discarded. A drag that is wide enough but very thin is expanded to 12 screen pixels high, centred on where you dragged, so a quick swipe along a line still produces a usable mark.
- Highlights do not snap to text. For a sentence that wraps, drag one box per line, or one box around the whole block.
- Colour and opacity can be changed per highlight after selecting it. The Text tool adds typed notes and the Rectangle tool adds unfilled outlines for callouts.
- Transparency is not allowed in PDF/A-1. Highlighting a PDF/A-1 archive copy breaks that conformance, while its metadata can still claim it.
Removing a highlight at each stage
- Before saving: select the highlight and delete it, or use Undo, which keeps the last 50 steps.
- After saving: restore an earlier version. Each Save keeps the previous copy, up to 20 versions, until the hourly cleanup removes files more than 24 hours old.
- After downloading: the highlight is part of the page. Covering it with a white box would hide the words underneath as well, so go back to an unhighlighted original instead.
Signed, restricted and scanned files
Saving rewrites the whole file, which breaks digital signatures. On a signed test file whose signature pdfsig reported as valid, a single highlight and save led pdfsig to report "Illegal values in ByteRange array" and leave the signature unverified. Highlight a copy for review and keep the signed original for the record.
Files with permission restrictions but no open password are refused at upload, because pdf-lib saves them as a damaged copy; in a test with an AES-256 restricted file, Poppler could not open the result. Remove the restrictions first with the owner password. Scanned pages need no special handling, because highlights are based on area rather than on recognised text, though the scanned words still will not become searchable.
The upload uses one check. Saving and downloading do not use more, and the download is named after the original with _highlighted added.
Highlighting in a desktop reader or Chrome instead
Every common reader can highlight text: Adobe Acrobat Reader, Apple Preview, the PDF viewers built into Chrome and Edge, and Firefox. They add true highlight annotations that snap to the selected words, carry your name and date, appear in the comments panel and can be deleted by the recipient with one click. When the person receiving the file needs to know who marked what, or needs to clear your marks later, use one of those readers. They are also quicker for marking many passages in a long document, because you select words instead of dragging boxes.
The drawn highlight from PDFCheck is the better choice in three situations: scanned pages and photographs, where a reader finds no text to select; files that must look identical in every viewer and on paper, since annotation display and printing vary by reader and settings; and a quick mark from a phone or a work computer where nothing can be installed.
There is no everyday command-line highlighter. The closest is stamping a page of colored boxes, drawn in another program, over the original with qpdf: qpdf report.pdf --overlay marks.pdf -- highlighted.pdf. That is only worth setting up when the same marks go on many files.
Before you upload a file to highlight
- Why was my file refused before I could mark it? Two checks run on upload. A file with an open password is turned away with "This PDF is password-protected", and a file with owner restrictions gets a longer message explaining that saving would remove them. Both need the password tool first. A file above your plan limit, 50 MB on Free and 100 MB on Pro and AI, is refused as well.
- Does each save use a daily check? No. The single check is spent on upload; Save, Restore and Download spend nothing. After 20 saved versions the editor stops with "You have saved 20 versions, the maximum" and asks you to download the PDF and open it again.
How long does my file stay online?
The upload and every saved version stay until the hourly cleanup removes files older than 24 hours. Your recent files are listed only in the same browser session. After the cleanup, opening one shows "File not found or expired".
Last updated . How these results were produced: the pdf-lib 1.17.1 drawing call the highlighter makes in the browser was replayed in Node.js on synthetic sample PDFs, including one signed with a self-signed test certificate, and the outputs were inspected with Poppler pdfinfo, pdftotext and pdfsig 26.10 and qpdf 12.4.