One memorandum, two memoranda. “Memorandum” is a Latin neuter (it means “a thing to be remembered”), and Latin neuters in -um change to –a in the plural. So this is wrong:
Wrong example: The notebooks were filled with memorandum to himself on things to look further into…
Change it to: The notebooks were filled with memoranda to himself on things to look further into…
Some spelling checkers (including the one the Editor is using right now) recognize “memorandum” but not “memoranda,” suggesting “memorandum” as a correction. Hold fast to your principles and ignore your spelling checker.
“Memorandums” is also possible as a plural, but might get you sneers from the pedants. “Memos” is shorter, simpler, and more friendly. “Notes” is shorter, simpler, and English.
UPDATE: Although the Editor wrote that “memos” would be a good short form for “memoranda,” second thoughts have suggested to him that “memo” in American English has a specialized meaning: it tends to make us think of a message circulated in an office, rather than something written down to be remembered later. “Notes” is probably the best short alternative to “memoranda.”