As the heading suggests, I'm wondering whether the abbreviation "PM" can be used for "pro memoria". I've looked it up in a number of dictionaries, but none of them list 'pro memoria' as a possible meaning of "PM".
It is listed at Abbreviations.com along with 212 other meanings, including Private Message, Prime Minister, Project Manager and many more of varying usefulness. (The abbreviation of Post Meridian is usually written in lower-case letters.)
The Guardian Style Guide says,
If an abbreviation or acronym is to be used more than once in a piece, put it in brackets at first mention.
So perhaps you should write "Pro Memoria (PM)" on its first appearance.
PM isn't included in this list of Latin abbreviations:
However, p.m. does appear in these examples:
Some of the Linguee sentences first introduce it with an explanation, which makes sense to do.
the proposal specifies p.m. (pro memoria).