I have commented for a a long time that people should not use DOC (or, more recently docx) format for mail attachments, in part because DOC format is a major vector for malware. This is still true. It has been drawn to my attention, though, that the same is also true of PDF attachments.
They don't all warn you. Some security firms are advising to open pdfs in google-docs. This will help a bit, since the pdf opens on google's computers and google sends you the results for your browser to display. Until someone figures out how to get the executed code to run in your browser (or use it to attack google.)
Narrow featured readers, like xpdf (gnu/linux platform) are not affected because they do not implement these features. Bear in mind that all software has bugs, and free software is no exception. You do need to keep your applications up to date.
So what of email attachments? It is very seldom that you need to send the information in an attachment at all. Put it in the main body of the email as plain text and send it as a plain text message. Nobody ever got a virus off ascii.