'Pamphlet' comes from 'Pamphilus' — a wildly popular 12th-century love poem circulated as a small booklet.
A small booklet containing information or arguments about a single subject.
From Anglo-Latin 'pamphiletus,' a diminutive pet-name for the 12th-century Latin love poem 'Pamphilus, seu de Amore' (Pamphilus, or Concerning Love), which was so widely copied and circulated as a short unbound booklet that its title became the generic term for any small unbound publication. The proper name 'Pamphilus' comes from Greek 'Pamphilos' (beloved by all), from 'pan-' (all) + 'philos' (loving, dear), from PIE *bhil- or *gwil- (loving, friendly). It is unusual in etymology for a literary character's name to become a common noun
A medieval Latin love poem was so popular that people stopped calling it by its name and just called it 'a pamphilus' — the way we might say 'hand me that Kleenex.' From there, any small unbound text became a 'pamphlet.' One fictional lover's name became the English word for an entire format of publishing.