The reason why you can't open your PDFs is because your Outlook's temporary memory (cache) is full. Here are the steps to clear this.
Step 1. Close Outlook
Step 2. Click on the windows Start button and type shell:cache and press enter:
Step 3. Double Click on the folder Content.Outlook
Step 4. There will be a folder (possibly more) made up of random characters. Something like 02CVZE8G. If there is only one, open that folder. If there is more than one folder there, only one of them will have any contents in them, and you will need to open that folder.
Step 5. With the folder open, delete all files. If you press ctrl+A, it will select all, then you can delete using the delete key. You can then close the window.
Step 6. Re-open Outlook and attempt to open the attachment again.