When the PDF Highlighter seems to misbehave, the cause is usually one of two things: something stopped the tool from running, or it ran but the result is not usable. This guide starts with the fastest checks, then walks through the common problems and exact fixes.
Quick Checks First
- Refresh the page - a stalled session is the most common cause of nothing happening.
- Confirm the input is not empty and actually contains what you meant to submit.
- Try the smallest possible input; if that works, the issue is size, not the tool.
- Turn off browser extensions that block scripts, then reload and test again.
- Update or switch browsers if the problem repeats across sessions.
Common Problems and How to Fix Them
I cannot select the text
Scanned pages lack a text layer - run OCR first, then highlight.
Highlights disappeared in another app
Flatten annotations before sharing so every reader displays them.
Everything is highlighted
Highlight with intent - mark only what you will actually revisit.
Colours mean nothing to my team
Agree a legend up front: yellow for key points, green for actions, and so on.
The highlight sits offset from the text
Re-select precisely - highlights anchor to text runs, not free areas.
I need to remove one
Select and delete the highlight before exporting - they stay editable until flattened.
When It Works but the Output Is Poor
If the tool runs fine but the result misses, the problem is almost always the brief, not the software:
- Highlighting everything and losing signal
- Using colours without a legend
- Highlighting images or scans with no text layer
- Assuming highlights are visible in every reader
How to Stop It Happening Next Time
- Keep a saved prompt template for every task you repeat.
- Change one setting per generation so causes stay obvious.
- Run a two-minute sanity test with known input after any update.
- Note every failure and its fix; a short troubleshooting log pays for itself.
Still Stuck?
Reproduce the problem with the smallest possible input and note the exact steps that triggered it. A one-line description of what you did, what you expected and what happened is enough to get a precise answer from support or a colleague.
Frequently Asked Questions
Do highlights change the document text?
No - they layer over the content, which remains untouched underneath.
Can I highlight scanned documents?
After OCR creates a text layer - otherwise there is nothing to anchor the highlight to.
Will my highlights show elsewhere?
Standard annotations display in most readers; flatten them when certainty matters.
Is my file uploaded?
No - highlighting runs entirely in your browser.
Related Guides
- How to Use PDF Highlighter
- What Is a PDF Highlighter and How Does It Work?
- PDF Highlighter Examples: Real Scenarios and Results
- PDF Highlighter for Students, Office Workers and Freelancers
- Best PDF Highlighter Alternatives Worth Trying
- PDF Highlighter Tips and Best Practices
- Is PDF Highlighter Safe? Free vs Paid
- More PDF editing tools and guides