This type of thing is often difficult to diagnose. If there's something wrong with the file, Reader will attempt to correct it if possible, which causes the contents to change, which causes the validation of the digital signature associated with the usage rights to fail. The source of the error is often related to fonts. There also seems to be forward compatibility problems, so it is best if you can enable the document with the most recent version of Acrobat, which is currently 11. Without seeing the file it's difficult to say what's wrong.
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