In order to disable the PDFM Add-in, you need to delete or unregesiter the DLL (or uninstall Acrobat)- you can't disable it from within Outlook.
Go
here for all of the removal options
I deleted the DLL using following these steps:
To delete PDFMOutlook.dll, follow these steps:
1. Close Outlook and Adobe Acrobat
2. Open Windows Explorer, and navigate to
“C:\Program Files\Adobe\Acrobat 7.0\PDFMaker\Mail\Outlook”
If you own another version than Adobe Acrobat 7, your path will differ in this version number
3. Delete PDFMOutlook.dll if it exists