I've tried both Firefox and I/E. I have both Adobe Acrobat Pro and Reader, and I downloaded the latest version of Reader.
Here's an image of want happens when I try to open a .pdf in my browser:
I've gone to Firefox and looked for an Acrobat plugin for the browser, but it just links to the Adobe download page for Acrobat. I really don't know why it's doing this. It was fine, and one day it just stopped working.
Did you tried following what's there on the error dialog? Sometimes, the Acrobat application process might be running even though the application is closed. It's better if you shutdown, restart your system
Yup, restarted my system. First thing I did was open Firefox and try to open a .pdf.
There isn't an Acrobat icon in my system tray, and it's not showing up in my task manager, so is there another way of seeing if it's open?
Ok, I found the fix (thanks to the related topics on the right). Having both Acrobat and Reader on the same system creates issues like this, and it is recommended to uninstall one of them. I uninstalled Reader, and everything works perfectly now.