PDF locked for editing. Mac users are reporting they cannot sign the document.

We create contracts for our Association members through Adobe Acrobat. We lock them with a password so that only fillable fields and signature fields are editable. 

However, we've found that Mac users are constantly having issues with the file, where the error "User Password Required" comes us everytime they try to sign it or fill out the date. 

Is this a common issue with Mac and are there workarounds?