A security company has alerted surfers to a worrying trend of malware attacks using PDF files.

F-Secure, a Finnish security firm, has warned of a “high volume” of emails containing infected PDF (portable document format) files, a popular format used for sharing documents that contain text and graphics.

Many attacks that use infected files attached to an email are picked up before they arrive in the recipient’s Inbox as they contain .exe, or executable files, which once opened install themselves on a PC.

The attacks are different inasmuch as PDF files are normally considered to be safe by most email-filtering technologies, so the emails will get through. PDFs are generally viewed using Adobe’s free Acrobat Reader software.