You can edit your PDF documents stored in your Sync account from your iOS device and save them back to Sync.
To Open and Edit the PDF Document:
Open the PDF document from the Sync iOS app
Tap on the Action (Share) button from the top right corner of the screen and select Copy to PDF Expert
The document will then open on PDF Expert where edits can be made
To Save and Update in Sync:
Tap the 3-dot button on PDF Expert located at the top right corner of the screen and tap Share
Choose either Document (to send file and retain editable annotations, signatures or fields) or Flattened Copy (to send the file to disable further editing on the document)
Tap Save to Sync and select the location where you want to save the file
Tap Add from the bottom right corner of the screen
Note:This will not overwrite the existing file. You will have to delete the older version of the file.