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PDF Based Phishing Scam
Basically, the user will receive a PDF file that appears to be locked. Once open, the PDF will ask for the email address and password. Once entered, the email address and password is forwarded to the hacker(s).
This is a very unsophisticated ploy, however, caution should be exercised if you receive a PDF that appears to be locked or encrypted, even if you think you recognize the sender.