Fix "Cannot Highlight Text" in PDF — Highlight Tool Not Working
When the PDF highlight tool selects nothing or highlights don't apply, the text is an image (scanned PDF) or the file has restriction permissions. Here's how to highlight scanned and locked PDFs.
"Cannot highlight" in a PDF — where clicking the highlight tool and dragging over text does nothing — has two distinct causes: the PDF is scanned (the "text" is actually an image) or the PDF has annotation restrictions enabled. Each needs a different fix.
Determine If It's Scanned or Restricted
Test: try to click and select a single word with the selection tool (arrow cursor). If the cursor selects individual words as text, the file has selectable text and the highlight tool is likely restricted by permissions. If the cursor grabs the entire page area instead of individual words, the PDF is scanned and has no real text layer to highlight.
Fix for Scanned PDFs — Add a Text Layer First
You can't highlight what isn't text. For scanned PDFs: upload to Google Drive → Open With Google Docs (which runs OCR), then download back as PDF. After OCR, the text is selectable and highlightable. Alternatively, in Adobe Acrobat Pro: Tools → Scan & OCR → Recognise Text to add an OCR text layer. Once the text layer exists, the highlight tool works on the recognised text.
Fix for Restricted PDFs — Remove Annotation Restrictions
If the text is selectable but highlights don't stick, the PDF has "commenting: not allowed" in its permission settings. Check: File → Properties → Security in Adobe Reader. If "Commenting: Not Allowed" appears, use FixMyPDF's unlock tool to remove the permissions restriction. After unlocking, open the file in Adobe Reader — the highlight and comment tools will be fully functional.
Highlighting in Browser Viewers
Chrome's built-in PDF viewer doesn't have a native highlight tool (as of 2024, though Edge does). For highlighting in Chrome: install a browser extension like "PDF Highlighter" or "Kami" that adds annotation tools to the browser PDF viewer. Alternatively, download the PDF and open in Adobe Acrobat Reader, which has a full highlight toolbar. For long documents you annotate regularly, Reader is more capable than any browser viewer for markup work.
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