Without further details, I would guess that some of them mey have got corrupted, not such an infrequent occurrence. Just in case, I would suggest that you ask for those files to be sent again, but in compressed form (zipped).
PDF files are extremely reliable. You may have a software issue. I have the problem on 2 identical systems with the exact same specs. The problem occurs at the same exact interval. It may be a windows update. We have had to re-install software form the DVD in each instance. This fixes it for awhile and then it recoccurs. Adobe acts as if they have never heard of the problem...imagine that. I'm the only one with th problem.
This is happens with CS6 volume lic product.
If you dont own Adobe product it will be even more difficult to find and answer as the reader is used my so many people.
We know we have the problem again when we double click opn a PDF and nothing happens. SOme have reported that reloading does not fix the problem.
Wish I could help you. I am looking for answers to this issue right now and phone support is a 30 minute wait.
Please let me know the following details:
1. The version of Adobe Reader on your system (You can get it from Help > About Adobe Reader)
2. Your OS.
3. Do you recive any error of sorts while opening attached PDFs.
4. Are you opening the PDF from your web based email account or via outlook.
5. Are you able to open PDFs stored locally on your system.