"small, unbound treatise," late 14c., pamflet, "brief written text; poem, tract, small book," from Anglo-Latin panfletus, which probably is a popular short form of "Pamphilus, seu de Amore" ("Pamphilus, or about Love"), a short 12c. Latin love poem popular and widely copied in the Middle Ages; the name from Greek pamphilos "loved by all," from pan- "all" (pam- before labials; see pan-) + philos "loving, dear" see -phile).
Meaning "brief work dealing with questions of current interest; short treatise or essay, generally controversial, on some subject of temporary public interest" is from late 16c.
noun word-forming element meaning "one who" (operates, produces, deals in); Englished form of French -ier, from Latin -arius, -iarius; compare -ary. Usually in English words of more recent borrowing from French; older words tend to keep -ier.
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Harper, D. (n.d.). Etymology of pamphleteer. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/pamphleteer