Replacing a file

I am trying to upload a new PDF document onto an existing submission, to replace a file already saved. 

so select the submission, edit, upload file, browse files, select applicable and submit. 

However when i return into the file my required file hasn't uploaded and it is still showing the old one? 

How can i replace a PDF? 

If I understood you correctly you are trying to edit your submission and replace already uploaded PDF file with new PDF file. However when you upload new PDF file and submit the form, the submission is still showing old file.

I have created sample form and I was not able to replicate mentioned issue. When I edited my submission I was able to replace old PDF file with new one. 

Can you please provide us ID of your form so that we can clone it and test it?

Also if by some chance both PDF files, old and new, have the same name try to change name of the new file to see if that will resolve your issue.