Upvote:4
The invitation letter should be original (as pointed out in their guidance) with 'wet signatures'.
This is especially true if it's the applicant's first visit. They will (or may) check the font and resolution of the letter.
You didn't mention the contents of the letter, but given the applicant is in Morocco, the letter should be fairly high quality and address all of the concerns they expect from a UK-based host.