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are you sure about "unprotected",
in my own experience , mostly I download a spec sheet, user manual, form to fill in, it is "secured", I use PDF exchange & get a visual confirmation of ( SECURED) on the title line.
As a quick test I just opened two specs/guides that I have to hand from Yamaha & Sony - both are secured.
don't confuse secured ( can't edit) with password protected ( password needed to open ) - they are different features
Some at least of the "it doesn't work" reports will trace back to calibre giving up, without an error message, if asked to edit a secured document
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