Is there a way we can use Acrobat Standard to assign a password for opening of signed PDF file once it has been downloaded from DocuSign?

Prior to implementing DocuSign, we used to scan manually signed document to a PDF file and then store the PDF file in a HR file share.  We would then use Adobe Writer Standard to assign a read/write password to the PDF file.  We now use DocuSign to have documents electronically signed and download the DocuSign PDF in our HR folders.  However Adobe Acrobat Writer does not allow us to assign a password.  It pops up a dialog box saying ""you cannot change security on this document because the document is signed or certified".  We want to continue being able to store the PDF in our HR file share with passwords to restrict who can view off the share.  Is there a way we can assign a password to the PDF file once it has been downloaded from DocuSign?