Android PDF.js by Manuel Reimer

Provides a way to view PDF files directly in the browser without cluttering up your "Downloads" directory. It uses Mozilla PDF.js to directly view PDF files. This is the same PDF viewer that is already built into the "Desktop Firefox" browser.

Top features:

• View PDF files directly in "Firefox for Android".
• No more need to fill up your "Downloads" directory just to preview some PDF files. You decide which PDF file is downloaded after having a preview.
• Displays PDF files embedded into websites.
• This Addon ships with a built-in version of PDF.js. No user tracking as no external PDF viewing web-service is accessed.

Note about privacy: It actually is possible to detect that you use my Add-on as it modifies websites to allow PDF display directly in your Android browser. A website could just check for these changes and use this information for fingerprinting. This is also the reason why this Add-on can not be enabled for incognito mode!

Important: Mozilla unfortunately does not allow developers to create Add-ons for the regular "Firefox" on Android. To use my Add-on, you have to install "Firefox Nightly" from the Play Store. Then follow the following tutorial https://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly/. My collection for Android can be found here if you don't want to create your own:
https://addons.mozilla.org/de/firefox/collections/5149765/android/
This is user id: 5149765
Collection name: android