The Challenge Magazine PDFs are excellent digital reproductions of this classic RPG magazine. The page and cover scans range from good to excellent quality (the covers may show some signs of wear from the original), and all text is fully searchable. I just wish they had gone the extra step to bookmark them as well.
This particular issue has a wealth of information for the Twilight: 2000 and Traveller RPGs. One of the pages in this issue is upside down, but since it's the intro page for the JTAS section (with an image and very little usable text), it doesn't detract from the quality PDF very much.