There are several brands of free PDF readers available, however many of us who started with Adobe prefer its quirks over some of the others' quirks. But the Adobe PDF reader is so prevalent that the malware writers searched for and found a vulnerability.
From what I can tell, the malware doesn't just attach itself to a PDF file floating through the email system but actually has to be built into a program disguised as a PDF file. The bad guys send out spam with the disguised PDF file attached, and some unsuspecting email recipients download the disguised PDF files, then when they open it with the Adobe Reader an executable program goes to work.
However, there's a way to avoid it which is explained at the aforementioned Naked Security site. But it only works on Adobe Reader XI, not on earlier versions. Download Adobe Reader XI (11.0.01) (46.74 MB) at that link.
In Adobe Reader XI click "Edit," "Preferences," then "Security (Enhanced)." The resulting screen shows a new feature called "Sandbox Protections." The default has the "Protected View" turned off which, according to the experts, won't protect the user. So turn it on by clicking the "All files" button. See the screen shots of both Adobe X and Adobe XI readers, below to see the difference.