One approach, from what seems an immaculate publication from San Jose State University › faculty › gerstman {B. Burt Gerstman, D.V.M., M.P.H., Ph.D. Professor Emeritus}:
4: Probability ...
...
...
[p 2] Binomial random variables are discrete RVs of “counts” that
describe the number of “successes”
...
[p 4] Probability mass functions (pmfs) can be drawn as pmf
histograms.
...
[p 5, bottom] For additional instruction on pdfs see §5.4 in Basic
Biostatistics for Public Health Practice (Gerstman 2015, Jones &
Bartlett, Burlington, MA).
(1) This is one suggestion; you can probably find others. From the next nearest educational establishment, perhaps, with equally immaculate publications. There are usually in-house specifications (which are not universally binding ... merely university-binding).
(2) Note that periods are not used here in initialisms (these are not acronyms in the most usually accepted default sense of the word, as individual letters are read out; contrast NATO); this is standard practice in the UK for all abbreviations where there are not compelling reasons to add a full stop, and is catching on quite rapidly in the States. Stan Carey at WordPress offers advice on cases where periods are opted for.
(3) Note that capitalisation seems to follow arbitrary tradition rather than strict logic.
(4) The 'rule' used here is 'simply add a lower-case s to pluralise any initialism/acronym, no matter what the case is.' I'd go with this myself, but would reserve the right to insert an apostrophe where I felt it would aid clarity.