I am unable to open pdf documents and fill and sign and also I get "failed to fetch" errors when I try to save a file. I get "Error: We cannot process this file because it is an unsupported format or is password protected."
Sorry for the delay.
As the message suggests the file is either be XFA PDF document which is a different structure for a PDF file or the file is really password protected.
Is it possible for you to share the file with us? If the file isn't sensitive you may upload it to any cloud drive and share the link with us. We will take a look and let you know our findings.