The best way to keep a static link to a document that might change, is to link to the file instead of uploading the file to the system.

Your company may have an internal storage solution or you may use dropbox. Link the maintenance plan source work order to the http:// (web) address of the file. When the document is updated, replace the old file by the new file (make sure the file name is the same).

Because you link to that file name the work order will always point your users to the latest version of the documents.