When I send a shared review, several (but not all) users report that the Place Signature option is grayed out. This only occurs through shared review; if I email the pdf as an email attachment directly then they can sign it. When I use the attach file (to email) option within shared review those same reviewers report the same signature issue. I need to have all of these users to be able to apply digital signatures through shared review.
As mentioned above, that users are unable to sign the document shared through the "Shared Review", so the moment when you share the document through the shared review the document let locked and changes are restricted.
If you check the document properties (Right click on the file) under security tab you will see that you are no longer allowed to add signature.
This is a default behavior of the application, that can not be changed.