How can I determine if a scanned document is locked before I send it to someone who is trying to electronically sign it?

Hello: I work with Fannie Mae (FNMA) to put contracts together. We manually scan docs (using simple 4 in 1 scanning devices) as pdf files and email to FNMA. They electronically ratify the contracts w/their software. Lately, they've said that the contracts I have sent them are blocked. How does this occur and how can I tell if a document sent to me is blocked?  I realize that if someone uses software to manipulate a pdf file or if they use docusign on a pdf file and simply email it to FNMA (without printing it all out and scanning) it will be blocked.  However, I 'm talking about a manually scanned document (where we take original docs and simply scan them as 1 pdf file and email to FNMA to ratify).  Pls help me with this.  Thank you.