How do I open attached pdf files?

Try this. Google Yahoo, go to their main website, click on Mail and then log in through that portal. Then try to open your attachments. Bookmark this new page. If you compare the new URL with the one where you can't download the attachment you will find they are slightly different.

After a month of the same problem starting mid November 2013, I finally fixed this which was a problem on several but not all computers. I went into yahoo mail settings and clicked on Security and clicked on the option to always use email in "https" and suddenly after a month of misery, the pop up window popped up with the attachments when trying to open them. Interestingly after going back to Security, the option disappears from Security after choosing it. Incidentally, hard to find but this is also an official yahoo answer from yahoo but I fixed it prior to finding that out. They acknowledged the problem.