Weird file format while saving files
Open regedit and navigate to HKCR\.pdf. Make note of what the (Default) entry value is. For example, if you have Adobe Reader it might say AcroExch.Document.
Now, navigate to HKCR\<name of default value>, so in my example above I would navigate to HKCR\AcroExch.Document.
See what the value for the (default) key is. This is where your corruption should be. You can change it to something more familiar like Adobe Acrobat Document. Or use whatever identifier you want to use, depending on what application it is that is opening the PDFs.