While not a "PDF," the Russian-hosted wiki Wahapedia is the gold standard for digital rules. It compiles the entire 9th Edition Astra Militarum ruleset—Core Rules, Codex, FAQs, and Balance Dataslates—into a clean, searchable HTML format.
The physical Codex is a hefty tome, spanning over 130 pages. It is divided into several critical sections: Astra Militarum Codex 9th Pdf
In the grim darkness of the far future, the Astra Militarum—known to many as the Imperial Guard—stands as the indomitable backbone of the Imperium of Man. While Space Marines may win the glory and the headlines, it is the countless regiments of mortal men and women, supported by crushing artillery and armored columns, that hold the line against the horrors of the galaxy. While not a "PDF," the Russian-hosted wiki Wahapedia
For many competitive players, the PDF of the fluff (lore) wasn't needed. They just needed the stats. Battlescribe (using the "40k 9th Edition" data repository) contains the raw data files for the Astra Militarum. You can export your army list as a HTML or PDF file containing all your unit profiles. It is not the full codex, but it is functional. It is divided into several critical sections: In