PDF Text Layer Checker
Client-side · No upload · No account
A PDF that displays fine but won't let you highlight, search or copy a sentence is almost always a scan: a stack of photographs with no text underneath. Drop the file in to find out before you spend an evening trying to annotate a picture.
Drop a PDF here
The file is read in your browser and never leaves your device — no upload, no server, nothing stored.
Why a scan feels broken
Everything a reader does with text needs text. Highlighting selects characters. Search matches characters. The dictionary looks up a word, read-aloud speaks words, and flashcards are built from a selection. A scanned page has none of those, only pixels, so every one of those features has nothing to work with.
Lecture handouts and photocopied chapters are the usual culprits.Anything that went through a copier or a phone camera before becoming a PDF is a scan. Anything exported from Word, LaTeX or a publisher's typesetting has a text layer.
OCR is a one-time fix. Once a text layer is added, the file behaves like any other PDF in GemReader and everywhere else. Do it before importing, and the highlights you make are real text you can review later.
Questions
What is a text layer?
A PDF page can hold text as text — fonts and character codes a viewer can select, search and read aloud — or as a picture of text, which is what a scanner produces. The picture displays perfectly and does nothing else. OCR software adds an invisible text layer on top of the picture, which is why an OCRed scan can be highlighted and a raw scan cannot.
How does the checker tell the two apart?
It looks for font objects in the file. A PDF with text has at least one font; a scan has only image streams. When the file stores its objects in compressed streams the fonts are not visible without decompressing the whole document, so the result is reported as undetermined rather than guessed.
It says undetermined. What now?
Open the PDF on a computer and try to select a sentence with the mouse. If the text highlights, there is a text layer and everything works. If the cursor drags a rectangle over the page instead, it is a scan.
How do I add a text layer to a scanned PDF?
Run it through OCR. Adobe Acrobat (Recognize Text), the free ocrmypdf command-line tool, and most scanner apps do this and write a new PDF with the text layer in place. Do it once, before importing, and highlighting, search, dictionary lookup and read-aloud all work from then on.
Is the PDF uploaded to check it?
No. The file is read with JavaScript in your browser tab and never leaves your device.