Is this happening with all the files or just this one? Also, let us know your steps of sending the document, are you using Acrobat/Reader or Adobe Sign?
There is a possibility the file you are trying to send for signature is a secured pdf. And you don't have rights to makes changes to the file.
Please try using a different file and let us know if that helps.
Were you able to solve this? Was it just one file that was causing it?