How to remove copy-protection from public domain film?

The film project I'm currently working on (a film noir comedy short) involves using existing footage culled from old Hollywood movies now in public domain. In order to get the highest quality possible, I have purchased and ripped the video files from DVDs. But in one case the DVD version of a particular film was inferior to a newly remastered version available for download from Amazon, which I purchased. Only problem is when I try to take that video file into Premier Pro, AE, or any other video editor, I either get an error message or one that says "The file cannot be opened because it is under rights management".

How do I get around this? The film itself IS unquestionably public domain, but I guess they somehow copy-proofed their remastered version. I'm sure there are ways around this. I just don't know how. Any ideas?

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Matt Crunk
Fiveacre Films Inc.
Film and HD Video Production & Post Services