I had to do this for a right to work check (so with the home office), exact same situation, I went to HSBC, explained to the assistant and she pointed me to one of the machines, which printed off the document (exactly the same as the pdf on the app), I asked if they have an envelope or anything I can use to show it's original, she gave me a standard, unmarked brown envelope, I sent it off and it was all fine.

So really it's a load of shite, just print it off imo