When I opened a file as .pdf all of my files (including Internet Explorer!) changed to .pdf and now I can’t change them back, and I can’t open any files. What do I do?
Are you using Windows Vista or Windows 7? Sounds like a common bug that affects LNK files, i.e. desktop shortcuts. As Rob said, the LNK file extension was wrongfully re-associated to PDF files. This is easy to undo using a registry fix. Find Windows Vista fixes here and you will find fixes for Windows 7 here.
I also wrote an article about the issue:
How To Change & Fix Broken File Type Associations In Windows 7 (and Windows Vista)
try changing the extension as .html
A fuller description of what you did and what happened might help identify the fix.
My guess is that you just changed the file extension, that's to say the bit at the end so all files are now called [something].pdf instead of .doc .exe etc.
I'm not sure how that could happen and reversing it could prove a bit challenging as you need to know what to change it back to for each individual file. A restore point or backups may help.
If you have a restore point you can use that to go back without jumping through extra hoops. Otherwise make a restore point and then try one of the following to match your version of Windows.
can you use windows restore point to revert back to earlier state where everything was fine.
if you are in windows 7 try this
Can you be more specific in what you did and what happened?
In the meantime have you tried a system restore?