The most common PDFs circulating online are scanned copies of the original Al-Qadim boxed set (1992) and supplements like Arabian Adventures . While these are not written for 5e rules, a savvy Dungeon Master can convert them.
For the DM willing to do a little work, buying the PDF of the original 2nd Edition boxed set is essential. The lore is edition-agnostic; the descriptions of the Golden Oasis or the Ruined Kingdoms of the Islands remain some of the most evocative writing in the game's history. zakhara 5e pdf