>>>I do have pdf installed
Can we assume, from that statement, that you know the attachment is actually a pdf file rather than (say) an Excel spreadsheet or a PowerPoint presentation? (i.e. have you checked the file extension?).
Assuming that the file is definitely a pdf, it's possible that Windows has lost the information telling it which program to use to open it. Try the following:
Save the file to your hard drive.
Go to it's location and right-click on it.
Select 'Open with'
Even if Adobe Reader (or whatever other pdf-viewing program you use) is actually listed there, DON'T click on it straight away.
Instead click on 'Choose program'.
THEN click on the relevant program AND put a tick next to 'Always use this program to open this type of file'.
Hopefully the file will then open properly and (because you ticked the box I referred to above) Windows will have the correct information to be able to open all pdf attachments in the future.