What you will want to do to start with is clear out the Outlook Secure Temp folder. The easiest way to do this is the following steps:
1. Open a PDF attachment from an email
2. Go to File | Properties
3. Go to the Description tab and Click on the Link for File Location
4. Close the PDF
5. In the folder that was opened up, press Ctrl + A to select everything
6. Press Shift + Delete to delete everything from the folder
7. Reboot the computer.
I'm typing this from memory, so some of the wording might not be exact. The reasoning behind this is that if you open attachments directly from email often, the secure temp folder will fill up and effect performance. The correct way to open attachments from email is to save them to a location and open them from there. Let me know how it works for you!