Some or all PDF attachments from SFDC operations don't open after clicking the paperclip symbol.
The maximum size limit is was 2 mb. The functionality has been enhanced in SFDC version 1.9.0 and the maximum size limit has been increased to 5 mb.
If you are not in a position to upgrade, a workaround is to compress zip the PDF file to lower than 2000kb. Then, the PDF attachment will open.
If the issue continues to occur after reducing the PDF file size, please raise a new case online and reference the title of this article.