If Ministers undertake to place a copy of a letter, or extra information, in the Library of the House of Commons, s/he will say so, in pretty much exactly those words. It's a formal undertaking. Only ministers can deposit papers in the Library. These are called Deposited Papers and the Library has been receiving them in hard copy for many years.
They are not in the least secret, merely rather inaccessible and pretty often spectacularly dull.