How to Change Firefox’s Default PDF App on the Computer

PDF links in Firefox keep opening in the browser's own viewer, even though Acrobat, Preview, or another reader is already installed on your computer. Changing that is a settings tweak, not a reinstall.

Open about:preferences#applications, find Portable Document Format (PDF), and point the action at your app. If the reader is missing from that list, set it as the system default PDF handler first, then restart Firefox.

In this guide, I have covered:

Make Firefox Open PDFs in the App You Want

A PDF link opens in Firefox’s built-in viewer, and you’d rather it opened in Adobe Acrobat, Preview, or Okular. Nothing is broken. Firefox is doing exactly what it was told, and the instruction lives on one settings page.

The control you want is the PDF action under Applications. It lets Firefox hand every PDF to one app, or ask you each time instead.

Set the Default PDF Application in Firefox

You have a PDF reader installed and you want Firefox to use it for everything. That’s this method: one action menu, applied to every PDF you open from the browser.

1. Open the menu in Firefox and select Settings.
2. Scroll to General and find the Files and Applications section.
3. Locate the PDF options in that section.
4. Clear the checkbox labeled Open in the default application instead of the built-in viewer if it is enabled.
5. Open the application list for PDF files, then select Portable Document Format (PDF).
6. Open the action menu for Portable Document Format (PDF) and choose Use [your preferred app] (default).
7. Close the settings tab and test the change by opening a PDF link in a new tab.

In a hurry? Type about:preferences#applications in the address bar to land on the file-type list directly.

Firefox only lists apps your operating system registers as PDF handlers. If your reader is missing, install it first, set it as the system default, then come back to this page.

You’ll know it worked when a PDF link opens in your reader instead of the browser. If it still loads in Firefox, the action menu probably fell back to the built-in viewer, so set it again.

Use Open With for a Single PDF

Sometimes one PDF needs a different reader, or you’d rather decide file by file. Right-clicking gives you that choice without touching the global setting.

1. Right-click a PDF link in Firefox.
2. Select Open With… or Open File…, depending on whether the PDF is a link or a local file.
3. Choose the PDF application you want to use.
4. Select the Always use this app to open .pdf files option if it is available.
5. Open the PDF again and confirm that it launches in the selected application.

The system’s Open With dialog usually writes the same file association Firefox reads on its Applications page, so the two stay in step. Menu wording differs between Windows, macOS, and Linux.

Change the PDF Handler Through Your Operating System

Your reader is missing from Firefox’s list, and reloading the page doesn’t bring it back. That’s a system-level gap: Firefox can only offer apps your operating system already knows about, so fix the association outside the browser first.

• Windows: Open Settings, choose Apps, select Default apps, and find the PDF entry. Change it to the PDF app you want, then return to Firefox.
• macOS: Right-click a PDF file, choose Get Info, select the PDF app under Open with, and click Change All if you want every PDF to use that app.
• Linux: Use your desktop environment’s default application settings. For example, the xdg-mime command can set the default handler for PDF files.

Restart Firefox after changing the system association if the new app doesn’t appear right away. Then check Portable Document Format (PDF) in the Applications list again.

Mozilla’s Firefox support discussion about PDF handling is worth reading when a PDF preference refuses to stay put.

Make Firefox Ask Every Time

You rotate between readers: Acrobat for contracts, Preview for quick scans, the built-in viewer for everything else. If that’s your week, let Firefox ask at the moment you open each file instead of forcing one app on all of them.

1. Open about:preferences#applications in the address bar.
2. Find Portable Document Format (PDF).
3. Open its action menu.
4. Select Always ask where to open.
5. Test a PDF link and confirm that Firefox displays the application selection prompt.

TIP: The prompt costs one extra click per document. If you open the same kinds of PDFs every day, a fixed app is the faster choice.

Switch Back to Firefox’s Built-in Viewer

The built-in viewer has real advantages: it’s fast, it never leaves the browser, and there’s no external app to launch. Reversing the change uses the same page you started on.

1. Open about:preferences#applications in the address bar.
2. Find Portable Document Format (PDF).
3. Open the action menu and select the option for Firefox’s built-in PDF viewer.
4. Close the settings tab and open a PDF to confirm that it appears in Firefox.

If you’d rather reach that option from the address bar, the Set the PDF Viewer From the Firefox Address Bar guide covers the shorter route. And when Firefox’s settings look scrambled across the board, resetting Firefox’s browser settings can help, though a full reset is broader than changing one PDF handler.

Why the Firefox PDF Setting Keeps Reverting

You set Acrobat as the handler, and a week later PDFs are back in Firefox’s viewer. The choice lives in your Firefox profile, and a browser update that touches that profile can quietly reset it.

Repeat the Applications steps and check whether the action still reads Use [your preferred app] (default). If your app has vanished from the list entirely, fix the operating system association before you try again.

Losing other data at the same time points to a profile problem rather than a PDF one. The steps for recovering Firefox data missing after an update explain where to look.

Check That the Change Worked

The settings page can look correct while the actual handoff still fails. Test with a real PDF instead of trusting the menu.

1. Open a PDF link or double-click a local PDF in your browser or file manager.
2. Wait for the document to open.
3. Confirm that the selected application appears instead of Firefox’s built-in viewer.

If the file still opens in Firefox, check the PDF action at about:preferences#applications, confirm the chosen application is installed, and verify the system’s default PDF association. When the reader does launch, you’re done: the setting holds until an update or a profile change interferes.

RELATED: View Site Info and Set Site Settings in Firefox on the Computer