Unable to compare PDFs

Hey all,

I'm unable to compare two PDFs that i'm working on. I'm getting the following error:

These documents have security settings which prevent comparison.  Please modify the security settings to allow editing and try again.

I then attempt to modify security settings and get the following error:

You cannot change security settings on this document because the document is signed or certified

Everything that I've come across on the community pages (albeit very old posts) point to this being possible, given the PDFs current security settings.

Both PDFs have the same 'document restrictions summary' below. Does anyone know which of these needs to be allowed in order to compare the pdf's or is it simply the "signed/certified" bit making this not possible?

Thanks all,