Firefox has a built-in PDF viewer that's gotten significantly better over the past two years. Here's what it can actually do — and when to use a dedicated PDF app instead.
What Firefox's PDF Viewer Handles
Firefox renders PDFs natively using PDF.js (Mozilla's open-source PDF renderer). For standard documents, it works well:
- Text PDFs: read, search, zoom
- Forms: fill in PDF form fields, save as PDF
- Signatures: basic annotation support
- Print: print to paper or save to a different PDF
- Annotations: highlight text, add notes (as of Firefox 120+)
Annotations (New-ish Feature)
Firefox 120+ added annotation support directly in the viewer:
- Text highlight: Select text → "Highlight" button appears
- Freehand drawing: Click the pen icon in the toolbar
- Text notes: Click the text icon → click anywhere on the page to add a sticky note
These annotations are embedded in the PDF when you save. Other PDF readers that support standard annotations will show them.
Where annotations live: They're stored as standard PDF annotation objects, not Firefox-specific metadata.
Form Filling
PDF forms work well in Firefox. Interactive fields (text boxes, checkboxes, radio buttons, dropdowns) are all editable.
After filling: File → Save As → saves the completed form as a PDF with your filled values.
Limitation: XFA forms (Adobe's proprietary form format, common in government/enterprise PDFs) don't render correctly. Firefox shows a warning and suggests a dedicated viewer.
Searching PDF Content
Ctrl+F opens the in-PDF search bar. Works on text PDFs; doesn't work on image-only PDFs (scanned documents without OCR).
Bookmarking Location in a PDF
Firefox supports PDF named destinations. When you navigate to a section via the document's internal table of contents, the URL updates to include the destination anchor. You can bookmark that URL.
Example: file:///path/to/doc.pdf#page=42 opens directly to page 42.
Display Settings
In the PDF viewer toolbar:
- Page layout: Single page, vertical scrolling, two-page spread
- Zoom: Fit to page, fit to width, custom percentage
- Rotate: Rotate 90° for landscape-scanned documents
Presentation mode: Click the icon at the far right of the toolbar for a fullscreen presentation view of the PDF.
PDF.js Settings (about:config)
A few preferences control the viewer behavior:
pdfjs.defaultZoomValue — Default zoom when opening. Options: page-fit, page-width, or a percentage like 100.
pdfjs.spreadModeOnLoad — Default spread mode: 0 = no spreads, 1 = odd spreads, 2 = even spreads.
pdfjs.scrollModeOnLoad — Default scroll mode: 0 = vertical, 1 = horizontal, 2 = wrapped, 3 = page.
pdfjs.enableHighlightEditor — Enable the highlight annotation tool (default true in Firefox 120+).
When to Use a Dedicated App Instead
Firefox's PDF viewer is good for reading and basic annotation. Use a dedicated app for:
- XFA forms — Common in government and enterprise documents
- Heavy annotation workflows — Multiple layers, custom stamps, signature signing
- PDF editing — Merging, splitting, page reordering (Firefox can't do this)
- PDF/A compliance — If you need guaranteed archival format
- Scanned document OCR — Firefox doesn't perform OCR
On Linux: Evince (GNOME), Okular (KDE), or MuPDF for lightweight viewing. For editing: LibreOffice Draw can open and modify PDFs.
Opening Local PDFs
To open a local PDF: drag it into Firefox, or File → Open File, or navigate to file:///path/to/file.pdf in the address bar.
Firefox also handles PDFs linked from web pages — it renders them inline rather than downloading (unless you've changed that behavior).
To change download behavior: Settings → General → Applications → find "Portable Document Format (PDF)" → change from "Open in Firefox" to "Save File" or "Always ask."
Firefox's PDF viewer is production-ready for everyday document reading and form filling. For annotation-heavy workflows or complex PDFs, a dedicated app is still better — but for most day-to-day use, you don't need to install one.
Top comments (0)