Here is what I do. A transaction usually has a Drag/Drop box and a "Show existing" link. I click it but do not see the document that I just snapped with a "Receipt snap" in QB mobile app. This is because QB knows better what this receipt/document is for and therefore will not let you use it the way you want. If you snap a receipt then it must be an expense, so I create a dummy expense for it. Then I go to my transaction, click "Show existing", and then switch from "Unlinked" to "All" at the top. Now the snapped document shows up in the list and you can attach it. After attached, you can remove the dummy expense and the receipt itself from the Receipts list.
I admit that it is probably easier to take a picture of the document with your phone instead of using the "Receipt snap", then upload the image to some cloud service accessible on the PC, then download it, then Drag/Drop the file to the Drag/Drop box in the transaction. If anyone figured out a better way, I would like to know.
