The document is apparently a wedding order of service for Lady Edith and a character named Herbert (presumably Bertie Pelham, played by Harry Hadden-Paton). Fans will remember that Bertie asked Edith to a dance in last year's Christmas special.
The member of the public who found the paper decided to hand it into The sun, showing the fictional character's chosen hymns, prayers and vows. Unless there happens to have been another couple with the exact same names that got married there in July, of course