Finding a straightforward PDF download for 'Gideon the Ninth' is trickier than navigating the Ninth House itself, honestly. Muir's work is published by Tor, and they're pretty strict about distribution. You won't find an official, free PDF just floating around—that's not really how major publishers operate with new-ish, popular titles.
Your best legal bet is buying the digital edition from retailers like Amazon Kindle, Google Play Books, or directly from Tor's website if they sell ebooks. The file you get is technically a DRM-protected EPUB, not a PDF, but most e-reader apps can handle it. For a true PDF, some academic or library services might offer it, but that's a paid route through institutions. I know some folks desperately want an offline file, but supporting the author this way is how we get more locked tomb books.
I finally caved and bought it on Kobo last year. The formatting is solid, and it's nice having it permanently in my library, even if it's not a simple PDF I can toss around.