Hi Priscillia,
Thank you for your update,
The UWP framework has some limitation in editing or overwriting the PDF document saved in the assets directory or in any other location in the machine. We can only get the write permission over the documents in the AppData of the application being deployed. We have provided a sample in the below link which loads a PDF document from the AppData location, in this case the original document can be replaced with the modified document.
Sample link:
Document link:
AppData location:
Local drive:\Users\User_Name\AppData\Local\Packages\Pakage_Name\LocalState
Example:
C:\Users\John\AppData\Local\Packages\4b24d4e8-eaf3-4c50-9336-09da68323145_7s9x03r8y752r\LocalState
Demo video link:
Note: Place the PDF document in the AppData location to successfully run this sample.
Please let us know if this is helpful.
Regards,
Aravinth S