For official D&D PDFs, I highly recommend that you purchase them from the DM's Guild (www.dmsguild.com). You can also get a variety of professional-looking homebrewn content if you're interested.

A lot of the "high-quality scans" are actually pdf versions of the books made by the publisher, who have a pdf version before they produce a printed one. For homebrewn material, templates exist to allow others to create similar-looking content.

Official PDFs versions of the same books have the same quality, as usually only the publisher releases it. In the case of older books, some rescans may have been done in order to offer up a higher-quality version of the product, and then both of these versions can be found.

A lot of the "high-quality scans" are actually pdf versions of the books made by the publisher, who have a pdf version before they produce a printed one. For homebrewn material, templates exist to allow others to create similar-looking content. In the case of PDFs of books published prior to the widespread use of the pdf or its equivalent, it usually is a high-quality scan in which the pages are scanned individually so as to avoid the darkened section of the page caused by the binding. It may also be an already digital image, as formats such as png appeared during the same time as AD&D.