If you're not a fan of relying on browser add-ons and plug-ins, you'll probably want to enable a feature that can help you ditch at least one of them. Firefox now has a built-in PDF reader, and while it doesn't offer all of the same features that Adobe's version does, it's more than sufficient for most uses.
Before you get started enabling the PDF viewer, make sure you have the latest version of Firefox and you've disabled the Adobe PDF viewer.
To check the version: click the menu at the top left-hand corner and go to Help > About Firefox. This will check for updates and allow you to apply any it finds.
To disable the Adobe plug-in: open the same menu at the top of the Web browser and select Add-ons. In the new area that loads, click on Plugins and then the Disable button next to Adobe Acrobat.
Now to enable the PDF viewer:
Step 1: Open Firefox and type "about:config" in the address bar. You may see a warning about editing this area, accept it to continue.
Step 2: Search for browser.preferences.inContent from the box near the top of the about:config area. Once you find it in the list, right-click on it and select Toggle.
Step 3: Now search for pdfjs.disabled. After finding it, right-click and Toggle this setting as well.
That's all you need to do for the about:config area, so feel free to close that tab and start browsing PDFs without any need for an extra plug-in or add-on.
(Via How To Geek)