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Adobe Digital Editions sample library includes several PDF ebooks.
Aside from testing DRM removal, ADE installations are prone enough to errors and corruption on both ends that I keep a link to the samples. If I get errors from Kobo or Overdrive, I check the sample library. If sample downloads work, then the error's probably not on my end and I should try again later. It's usually not my end, but the test has caught a few times that I needed to delete my ADE credentials and reauthorize.
Each download request creates a new session (or whatever; you'll never get the "already fulfilled" or "too many downloads" errors), so I can test things like multiple ADE accounts without having to find another free ebook that I haven't downloaded yet.