Sorry for taking so long in responding to your query.
Are you still experiencing this issue? If yes, please try with another pdf file and check if this behavior still continues.
Also, let us know the application - Reader/ Acrobat and dot version of the application installed on the machine. To check the version in Reader/Acrobat: Identify the product and its version for Acrobat and Reader DC
Sorry we have just seen this response.
This issue is still happening and happens on all PDF documents.
This version I have installed is Adobe Acrobat Pro DC
Thanks! Would you mind sharing the dot version of Acrobat running on the machine? You can check the dot version by launching Acrobat > click Help > select About Adobe Acrobat Pro DC.
Also, could you please share one of the pdf files with us? You may refer to the steps given in this link on : How to share a file using Adobe Document Cloud
I have since had to install the following Adobe version due to another issue with Adobe Acrobat DC Standard.
I now have the following version installed and am still have the same issue.
Adobe Acrobat XI
I will upload a PDF document via the link.
We received your message about replicating the issue on the file you shared, "but it seems that the pdf file is signed or certified and because of that i was unable to change security settings on the document"
This is the exact problem we are having with ALL documents at our end. Something changed in our Adobe settings some time ago that we can no longer set security - password settings on our PDF's
This document has no signatures, BUT it has an AcroForm dictionary with a SigFlags entry of 2 (which means "Append Only"). I would not be surprised to discover that Acrobat is taking the existance of the SigFlags entry to mean that the document is signed, which it isn't (if it were, the SigFlags value would be 1 or 3). Possibly (probably) a bug in Acrobat. I don't know why there was a SigFlags entry in the first place. It is optional. That may be where the real problem lies. Did you change the file creator lately?
After writing the above, it occurs to me that the SigFlags entry of 2 would prevent a Full Save of the document. Encryption requires a Full Save (no incremental save sections). So the dialog didn't give the correct reason, but you can't change the security settings if you can't Full Save. Again, you need to check how the file was created (and modified, since there are several incremental save sections)
Thank you for the above response. There are 4 of us that all use single licence Adobe versions, 10, 11 & DC Standard we all started having this security issue at the same time, I'm certainly not aware of a change in the file creator. Is is possible our third party software has made a change in the way the reports are produced?
That's the most likely answer. The file shows the Author or Creator as "Merge Report", with a comment: "Created by Wnv/EP PDF Tools". It also claims compatibility with PDF-1.4, which is a very old (Acrobat 5.0) version. I can see no reason for a SigFlags entry, at all.