Noncommercial pamphlets (1), leaflets, and religious tracts have a long history, usually conveying urgent information. Most brief leaflets and religious tracts are freely distributed. Many pamphlets are more substantial in length and form, and they may have a modest purchase price. Sometimes they are distributed by hand in a public place. For example, an activist group might distribute a pamphlet about gentrification at a town hall meeting. With their history as street and protest literature, pamphlets, leaflets, and tracts may certainly be regarded as historical precedents for zines, sociopolitical zines in particular.
screenshot from the t.v. series The Essex Serpent (2022), showing someone selling pamphets, which were a means of communicating urgent news in England in 19th Century.
Thomas Paine's pamphlet Common Sense with its urgent message (and originally anonymous authorship) would probably be described as a zine if it were to be published today. Here's a description of the Radical and Labor Pamphlets collection, 1896-1967 at Duke University. Just the description, even, reveals so much about American History.
In the early 20th Century, Ida B. Wells published many pamphlets to educate Americans about the horrors of lynching.
However, pamphlets and related ephemera have been utilized for good and bad. Some of the most interesting (and appalling) pamphlets appeared right after the printing press was invented: pamphlets about the evils of witches. Pamphlets and related ephemeral publications have always represented the best and worst in people: leaflets published by racist organizations like the Ku Klux Klan vis-à-vis Wells's pamphlets to stop lynching are good examples of this binary.
The key distinction between pamphlets and ephemera and zines is the means of publication. There were no Office Max or Staples stores until late in the 20th Century, so with few exceptions, most people with something to say had to pay a printer to produce their pamphlet or leaflet. They couldn't actually do it themselves (DIY). Printers might refuse to publish something, or people might not be able to afford the cost of printing.
Some artists create pamphlets: they use that term. Someone else might describe the publications as zines or booklets. Just know that people describe publications in many ways, often based on personal preferences.
(1) "pamphlet." The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide, edited by Helicon, 2018. Credo Reference, https://search.credoreference.com/content/entry/heliconhe/pamphlet/0?institutionId=453. Accessed 06 Apr. 2022.