The Outlines of Pyrrhonism (Πυῤῥώνειοι ὑποτυπώσεις, Pyrrhōneioi hypotypōseis) is a work by Sextus Empiricus, written sometime in the second or third century CE. it is the main source for the ideas of the sceptical movement which traced its roots to Pyrrho of Elis, a philosopher who lived hundreds of years earlier in the 3rd Century BC.
The Pyrrhonists were opposed on the one hand to dogmatic philosophers such as the stoics, who believed they could attain certain knowledge, and on the other to the rival form of scepticism associated with the Academics, or followers of Plato.
Rediscovered during the renaissance, the work had a profound influence on modern thinkers such as Montaigne, Descartes and David Hume.
Free online texts
The availability of English translations of the Outlines in the public domain appears to be very limited. While some can be found online, their copyright status is often unclear at best. If you know of a link a comment below would be most welcome.
Gutenberg: Sextus Empiricus and Greek Scepticism by Mary Mills Patrick (1899), includes a translation of the first book of the 'Pyrrhonic Sketches'. Multiple formats.
Internet Archive: Sexti Empirici Opera, Greek text, edited by Hermann Mutschmann (1912).
Bibliography on Skepticism, by Diego E. Machuca.
History of Philosophy without any gaps: Healthy Skepticism: Sextus Empiricus - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Ancient Greek Skepticism, by Harald Thorsrud.
Philpapers: Outlines of Pyrrhonism - academic bibliography.
Stanford Encyclopedia of Philosophy: Sextus Empiricus, by Benjamin Morison.
The Partially Examined Life: Pyrrhonian Skepticism According to Sextus Empiricus - podcast.
Wikipedia: Sextus Empiricus.
Further reading at Tom's Learning Notes
Cicero: Academica - discusses the form of scepticism, distinct from Pyrrhonism, associated with the Platonic Academy at some periods.