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If you can open the PDF in Adobe Reader, go to Document Properties, security tab and look at "Security Method" if it says anything besides "No Security", it is encrypted (and DRM protected) using the named Security handler. Note: Adobe currently provides 4 security handlers (Password, Certificate, Live Cycle, and eBooks), and there are a multitude of 3rd party security handlers.
If you cannot open it in Adobe Reader, assuming it is a valid non corrupt PDF file, it is probably DRM protected.
If people are interested, I can give the true Geek answer of how to walk the PDF structure in a text editor to find out all of this information.
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