Try to select the text with your cursor. If nothing selects, there is no text there and OCR is the answer. If text selects but the highlight tool is greyed out, the document's permission flags are switched against you. If the highlight appears and then vanishes, it is being made but not saved.
And if none of those apply, the reader you are using may not offer highlighting at all, which is more common than people expect.
All four produce the same experience: you drag across a sentence and nothing turns yellow. What differs is where in the chain it stopped, and the chain is short enough to walk through in a few minutes.
Three quick tests that name the cause
Try to select the text
Drag your cursor across a line as though you were going to copy it. If a blue selection appears, there is real text on the page and the problem is further along. If nothing happens, the page is a picture and that is your answer.
Look at the highlight tool itself
Is the button greyed out, or does it look available and simply do nothing? A greyed-out tool is the document refusing. An available tool that produces no mark is the reader or the save step.
Highlight, close, reopen
Make a highlight, close the file without being asked anything, and open it again. If the highlight is gone, it was never written into the document.
Your reader may simply not do it
This is the cause people rule out first and should not. Reading a PDF and annotating one are different capabilities, and plenty of perfectly good readers offer only the first. A viewer built into an operating system or a browser is built to display documents reliably, and markup is a feature that gets added later, if at all.
The practical consequence is that the same document behaves differently depending on where you opened it, which makes the fault look like it belongs to the file. Opening the file somewhere else is the fastest test there is, and it costs nothing.
Adobe Acrobat and Reader, on desktop and mobile.
Preview on macOS, which has had markup tools for years.
Most dedicated third-party readers.
Browser-based PDF editors, including the one on this site.
Chrome's built-in viewer, whose markup tools are few and vary by version.
Many in-app viewers inside mail clients and messaging apps.
Document management web viewers, which usually render a picture of the page.
Some e-reader apps, which treat a PDF as a fixed image.
If you highlight regularly and you are fighting a viewer that was not built for it, the fix is not a setting. It is opening the document somewhere that treats annotation as a first-class feature.
There is no text to highlight
If dragging across the page selects nothing at all, the page is a picture of a document rather than a document. A scanner produces exactly this: an image of paper, faithful to look at and completely opaque to anything that wants to work with the words.
Highlighting requires something to attach to. The annotation records a region and the text it covers, so with no text objects on the page there is nothing for the reader to anchor a highlight to, and a well-behaved reader declines rather than guessing.
Optical character recognition solves it by reading the image and writing an invisible text layer behind it, positioned so that each recognised word sits over its own picture. The page looks identical and now selects, copies, searches and highlights. The four reasons text will not select covers the other possibilities, including the awkward case of text that selects but copies as nonsense.
If you only need to mark a scan for someone reading it, not search it, OCR is optional. The PDFslime PDF editor draws its highlight as a box at 35 percent opacity into the page content, so it works on an image page, appears in every reader and prints. The trade-off is that it is part of the page, so nobody can later remove it as a comment.
Add a text layer to a scan
Recognition runs in your browser and writes the words behind the image, so the page looks the same and finally selects.
The tool is greyed out
A greyed-out annotation button means the document is asking readers not to allow it. PDFs carry a set of permission flags covering printing, copying, editing and annotating, and mainstream readers honour them.
The file opened without asking you for a password. It may still be encrypted, but with a key any reader can work out on its own, so nothing is being kept from you. What exists is a request, recorded in the file, that readers disable certain actions, and a reader that chooses to comply. It is a convention rather than a lock, which is why clearing the flags with PDFslime's Unlock PDF produces a copy that annotates like any other document. The two kinds of PDF password goes through the distinction in more detail.
The same flags travel with the file forever and apply to everyone, including whoever set them and has since lost track of the password.
What a highlight is, and why it can vanish
A highlight is not a change to the page. It is a separate object stored alongside the content, a text markup annotation in the terms of the PDF specification, recording a colour, a set of coordinates and the words it sits over. The page underneath is untouched, which is what makes a highlight removable.
That layering explains the fourth failure. Because the annotation is a separate object, it has to be written into the file, and not every reader does that automatically. Some keep annotations in their own store until you save. Some mobile apps hold them in memory and discard them on close. And a document opened from an email attachment or a read-only location may be saving to a temporary copy that disappears with the session.
The test is the one in the diagnosis above: highlight, close, reopen. If the markup survives, all is well. If it does not, save the file explicitly to a location you control and check again. When the annotations have to reach somebody else intact, flatten them into the page content. They then become part of the page rather than a layer a reader can decline to draw, which is the subject of why comments go missing for the recipient.
Common questions
Why a highlight will not go on, and what each symptom means.
01Why can I not highlight text in my PDF?
Either the page is an image with no text on it, your reader does not offer highlighting, the document's permission flags disable annotation, or the highlight is being made and then not saved. The three tests in this article separate them.
02Why is the highlight tool greyed out?
The document carries permission flags asking readers to disable annotation. The file opened without a prompt, so any encryption on it uses a key every reader can work out without a password. That is why clearing the flags produces a copy that annotates normally.
03Can I highlight a scanned PDF?
Not with a normal text highlight until it has a text layer, which OCR adds. If you only need to mark the page for a reader, a drawn highlight works without OCR: PDFslime's editor paints its highlight as a translucent box into the page itself, so it sits on a scan like any other drawing.
04Why do my highlights disappear when I reopen the file?
They were never written into the document. Some readers keep annotations in their own store until you explicitly save, and some mobile apps discard them when you close the file. Saving a copy with the annotations flattened in makes them permanent.
05Does Chrome let you highlight PDFs?
Its built-in viewer has far fewer markup tools than a dedicated reader, and what it offers depends on your Chrome version and platform. Look for an annotate or pen button in the viewer's toolbar. If there is none, the viewer cannot highlight and no setting will change that.
06Will my highlights show up for everyone else?
Only if the recipient's reader draws the annotation layer, which most do but not all. Flattening the annotations into the page content makes them part of the page, so they appear everywhere including in print.