1) The Adobe DRM system has been broken on many occasions – see http://www.locklizard.com/pdf_security_news.htm
2) You have to protect the same document every time for every user which uses unnecessary processing power, disk space and bandwidth. With the LockLizard system you only protect the document once.
3) Adobe does not prevent screen grabber applications from taking screen shots.
4) Adobe DRM costs $65,000+. The LockLizard system costs $4995.
5) Adobe does not prevent print to file drivers so users can print “protected” PDF documents back into PDF files.
6) Adobe DRM is more suited to internal use. It uses either PKI (so users have to be issued with certificates) or passwords (which can easily be compromised).
7) Adobe DRM is expensive to maintain and administer.