Do my highlights show up in Acrobat?

Yes. PDF highlights are part of the standard PDF specification — they're drawn as filled rectangles with a multiply blend mode in the page's content stream. Acrobat, Preview, Foxit, browser-built-in viewers, and mobile PDF readers all render them correctly. They print with the page; they survive merge / split / extract operations.

Can I remove a highlight after adding it?

Yes — click the highlight to select it, press Delete or use the right-click menu. Cmd/Ctrl+Z undoes the most recent highlight if you change your mind. The original PDF you dropped stays unchanged in browser memory until you click Download, so closing and reopening resets to a clean state.

Will highlighting cover up the text underneath?

No — that's the point of multiply blending. The text below the highlight remains fully readable as a darker tinted version of the highlight color. If your highlight does feel too opaque, drop the opacity slider to around 30% in the format bar; the default 50% works for most documents.

Can I highlight a scanned PDF?

Yes — but the highlight will be a colored rectangle over an image of the text, not over the text itself. The visual result is identical (a yellow highlight over the readable area), but a screen reader can't announce the highlighted text as "highlighted" because there's no text layer to associate with. For scanned PDFs that need accessible highlights, run OCR first (v1.1 paid feature).

Do my files leave my browser?

No — at all, today. All editing runs client-side via pdf.js and pdf-lib. The v1.1 paid tier will add opt-in cloud OCR / unlock; even then files are deleted within 60 seconds.

Yes. Everything in the editor today (annotations, AcroForm fill, watermark, page numbers, split, merge, export) is free forever. Paid tier ($1.99/document) covers cloud OCR + AI autofill on scanned PDFs in v1.1.

How do I keep my work between sessions?

Edits autosave to your browser's IndexedDB every 500 ms. The editor URL becomes /app?s=… — copy or bookmark to return. Sessions live entirely on your device.

Can I use this for sensitive documents — tax, medical, legal?

Yes — that's the point of client-side execution. Your tax return, medical record, or legal contract literally never touches our infrastructure. Verify in DevTools' Network tab.

Does Vietnamese text export correctly?

Yes. The export pipeline auto-fetches Roboto-Vietnamese (with full diacritic coverage) when any annotation contains non-ASCII text.

Highlight PDF

Highlight any PDF online. Free, no upload.

Drag across the text you want to mark, pick from yellow / green / pink / blue / orange, adjust opacity. The highlight is rendered with multiply blending so the underlying text reads through as a tinted version of the highlight color — same as a paper highlighter.

Drop your PDF here

or click to browse · 10 MB max free · stays in your browser

Open the free editor How it works

Files never leave your browserNo signup requiredDrag-to-highlight · 5 colors · adjustable opacity · multiply blend

FREE TIER

$0 forever

Highlight any text · 5 colors · client-side

Open editor

TEXT-AWARE · v1.1

$1.99 / document

Word-level highlighting via OCR text-layer · ships with v1.1

See pricing

The full guide

Everything you need to know about highlighting PDFs online.

Why highlight a PDF?

Highlighting is the fastest way to mark up a document for collaboration without changing the source content. Three common workflows: marking the relevant clauses in a long contract before forwarding to a colleague, picking out the answer-bearing sentences in a study PDF, and flagging discrepancies in a financial statement during review. Unlike comments or annotations, highlights are visible at a glance — readers don't need to click anything to see what you marked.

How highlights blend with the underlying text

PDFCatalyst uses BlendMode.Multiply for highlights, which is the same compositing mode a real paper highlighter would use. The highlight color (yellow, by default) tints the underlying page rather than overlaying it opaquely. The text below remains fully readable as a darker version of the highlight color — no obscured words, no fighting between the highlight and the page background. Pick lower opacity (around 30%) for a subtler tint, higher (around 60%) for a stronger mark.

Five highlight colors and when to use them

Yellow is the default — neutral, expected, fastest to recognise. Green works well for "approved" or "important positive" markings. Pink and orange both stand out against typical page colors and are good for "needs attention" or "second pass". Blue reads as informational rather than urgent. Multi-color highlighting in one document is fine — Acrobat and every other PDF reader respects per-highlight color settings.

Highlights survive Acrobat editing

PDF highlights are real graphical objects in the page's drawing instructions, not annotations layered on top. That means anyone opening the file in Acrobat, Preview, Foxit, or any other PDF tool sees the highlights immediately, can't toggle them off, and can't accidentally edit the underlying text without also editing the highlight. The highlight scrolls with the page, prints with the page, and survives merge/split operations.

Privacy: highlighting runs in your browser

Server-side highlight tools upload the WHOLE PDF — including any pages you didn't touch — to draw a few rectangles on it. PDFCatalyst's highlight tool runs in your browser via pdf-lib. The source PDF is read into memory, the highlights are drawn into the page object's content stream in memory, and the resulting PDF is downloaded directly to your device. Zero network round-trip; verify in DevTools' Network tab.

How it works

Highlight PDF in 4 steps.

01
Open the PDF
Drop your PDF onto the editor at /app or click to browse from your device.
02
Pick the Highlight tool
Click the highlighter icon in the left toolbar (or press H). The cursor becomes a crosshair.
03
Drag to highlight
Drag across the text you want to mark. The highlight rectangle snaps to the drag area.
04
Adjust and download
Use the format bar to change color and opacity. Click Download to save the highlighted PDF.

Frequently asked questions

Questions about this tool.

Today, no — highlights are drawn as rectangles via drag, not as text-selection-aware spans. Text-aware highlighting is on the v1.1 backlog (it requires per-glyph layout extraction, which is heavier). For now the rectangle approach works well for highlighting whole lines or paragraphs; for in-line word-level highlights, draw a tighter rectangle around just those words.

Highlight your PDF in your browser. Free.

Drag across the text, pick a color, download. The highlights show up in any PDF viewer.

Open the free editor