In recently weeks, I have noticed an increase in the use of PDF files for spam. Instead of uploading an HTML page using a compromised account, as seen shown in a previous post "Hundreds of College and Government websites still redirecting to fake stores", spammers are instead uploading PDF files. My guess for their motivation, is that PDF files are less likely to be checked for spam than plain HTML pages.
Most of the spam PDF files contain text only.
Example of spam PDF
In this example, the user is redirected to hxxp://searchglobalsite.com/in.cgi?23 (the URL is obfuscated in the PDF file) which then redirects to hxxp://www.results-today.com/.
The list of websites hosting spam PDF files is very similar to what I have reported earlier. It includes college web sites, governmental sites, and wiki pages, such as the following: