Because the people who developed the application used did a bad job of it.
And a digital signature doesn't ensure that the file won't be edited, only
that it will be possible to detect that it was edited.
On Thu, Nov 16, 2017 at 8:14 PM, mir16225051 <firstname.lastname@example.org>
Ok, but there's a solution, another way to block the document in order to avoid any modification?
No, there is no effective way to stop modifications. This is why signing is so important, and training users to validate signatures (NEVER look at the stamp on the page, it was a terrible mistake by Adobe to put something on the page when signed, because people trust it).