The highlight tool in Adobe Acrobat Reader DC does nothing when I try to highlight text in a PDF, or highlights nothing. Some PDFs it works, some it does not.
Why does highlighting fail on certain PDFs?
Highlight tool not working in Adobe Acrobat Reader DC
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RK
Rachel Kim October 26, 2020
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The works on some PDFs but not others clue is the whole answer, because it points at the difference between PDFs with real text and PDFs that are images of text, which the highlight tool treats completely differently:
The core cause, scanned versus text PDFs: the highlight tool highlights actual selectable text, so it works on a PDF made from a document, where the text is real and fails on a scanned PDF, which is an image of a page with no selectable text underneath for the tool to grab. If highlighting does nothing on a specific PDF, test whether you can select text on it and if you cannot select the text, there is no text to highlight, which is the situation rather than a broken tool.
The fix for scanned PDFs, OCR: running OCR, text recognition, on the scanned PDF creates a selectable text layer over the image, after which highlighting works, since there is now real text to select. Acrobat's paid version has Scan and OCR, Recognize Text and free OCR tools or online services add the text layer too, converting an image only PDF into one with selectable, highlightable text. This is the actual fix for highlighting a scan.
The other causes on text PDFs, when text is selectable but highlighting still fails: a PDF with security restrictions that disallow commenting or editing blocks the highlight tool, checked in the document properties security tab and needs the restriction removed by the owner or a route around it for an authorized document. A Protected View or read only mode from the file being downloaded can also disable markup until you enable editing or unblock the file. And occasionally Acrobat Reader itself needs an update or repair for a tool glitch.
The diagnosis: try to select the text first, since selectable text that will not highlight points at security restrictions or Protected View while unselectable text points at a scanned image needing OCR, the single test that sorts the two fundamentally different causes. Most highlight tool failures on certain PDFs are the scanned image case, resolved by OCR adding the text layer the highlighter needs.
The ones that would not highlight were scanned PDFs, I could not select their text at all, exactly your diagnosis. Ran OCR on them and highlighting worked immediately afterward since there was finally real text. The try to select text first test instantly sorts which problem I am dealing with. Simple once explained.