Most of the PDFs circulating online are scanned by community members (preservationists) to keep the history of the game alive. While Games Workshop encourages buying their current products, there is a soft understanding among the community that archiving these out-of-print texts is vital for the hobby's history.
According to the Warhammer 40k Wiki, the Codex Imperialis was a comprehensive guide to the reorganization of the Imperium of Man’s political and socioeconomic institutions. In the context of the tabletop game, it provided the primary lore background and army lists for various factions before individual codexes became the standard. Where to Find the PDF and Modern Rules warhammer 40k codex imperialis pdf