Okay, it appears that our PDF Stamper does create a PDF for each buyer with security settings that may differ from the original in a predictable way.
Because of the way our stamper works as a PDF authoring program itself and creates a brand-new PDF document for each buyer, it has to re-apply any security settings itself and these apparently cannot be simply duplicated from the original source document on a case-by-case basis. Thus, it applies sensible editing restrictions that prevent actions relevant to piracy/plagiarism or removal of the stamp, while allowing other actions that some PDF sellers may actually ~want~ buyers to be able to perform.
Our PDF stamper does add a random author/edit password, which forbids document creation (adding/deleting/changing the existing content) and document assembly (adding/removing/reordering pages), which are the only editing capabilities relevant to piracy or plagiarism. Restrictions on printing or copy/paste actions can also be added y using the &pdf_allow_copy=n&pdf_allow_print=n parameters in our PDF stamping URL.
However, it also permits superficial editing actions which do not affect the original content, such as filling in form fields, adding comments, and adding a digital signature key. For instance, a buyer might be expected to type some data into some form fields provided in the file, add a digital signature key to prove it was really them who filled in the form, and then submit the completed file somewhere.
Thus, apparently the Adobe document status saying, "You can edit this document" is only referring to a limited range of ~some~ things which are permitted, rather than carte blanche to do anything at all to the document content.
