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PDFs act very differently depending on how they were made. It doesn't much matter while you're reading them in Acrobat Reader or ADE, but if you try to convert them to anything else, the technical side of things shows up fast.
The biggest difference is between image PDFs and text-based ones, and most scanned PDFs are just images; they don't convert well to anything because there's nothing to convert except a JPG or TIFF image.
If the original PDF was created on a letter-sized page with large margins to allow the text to fall into a book-sized area, and then cropped, the conversion program might keep part or all of the original page size, which looks like it might be happening here.
If you open it in Acrobat Reader, you can go under File-->Properties, and check the Description tab; it says what program made the PDF. That's not always useful info, but sometimes warns of potential problems.
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