The PDF looks blank but selecting text highlights something. Or the search finds matches on a page that appears empty. The text exists — you just can't see it.
What's actually going wrong
White-on-white. Text is set in white colour on a white background. Selectable, searchable, but invisible. Transparency at zero opacity. Text exists with 0% opacity, technically there but completely transparent. Wrong colour space. Text colour falls outside your viewer's renderable range.
The quick fix
Convert PDF to Word. The conversion reads the underlying text regardless of visibility settings and produces a Word document where the text is in standard black. Confirm content is what you expected, export back to PDF with normal colours.
The new PDF has the previously-invisible text now properly visible.
If that didn't work
If the text contains intentional hidden content (commentary, redaction marks), recovering it might not be appropriate. Be sure the visibility issue is unintentional before fixing.
For PDFs with overlaid invisible text (common in OCR'd documents — invisible text layer over visible image), the invisible text isn't meant to be visible; it's there to enable search and selection while preserving the image's original appearance. That's correct behaviour, not a bug.
Prevent it next time
When designing documents, set colours explicitly. Don't rely on defaults that might land on white. And test PDFs by opening them after creation — invisible text shows up immediately when you try to read it.
FAQ
Is invisible text in a PDF a security feature?
Sometimes — OCR text layers are invisible by design. Redacted content shouldn't be invisible-but-recoverable; that's a redaction failure (see articles on redaction).
How can I tell text is there if it's invisible?
Try Ctrl+F to search for known content. Try drag-selecting on what looks like a blank area. Either reveals invisible text.
Will OCR turn invisible text visible?
OCR reads visual output, so if text is truly invisible (white-on-white), OCR can't see it either. Conversion to Word reads the underlying text data regardless of visibility.
Can I selectively reveal invisible text?
Tools that edit PDFs at the structural level let you change text colour properties. Easier route: convert to Word, set colours there, re-export.
Invisible-but-present text comes back via Flint's converter. Convert, recolour, re-export.