You say 1.9MB, the Gutenberg page says 1.8MB, and the download itself is only 1.18MB thanks to gzip. The main difference is that the Gutenberg PDF is a transcribed, cleaned-up version.
As much as this probably makes me sound like an audiophile, I actually prefer the raw scans over what may essentially be a reprint. They show all the blemishes, unofficial additions, and other marks that make the book look more "real" and give it character.
In this instance, the raw scan has a picture of the cover, as well as an interesting note handwritten near the beginning: "Property of Edward M Sumner" with an address. IMHO these sorts of historical artifacts are worth preserving too. I've come across scans with random notes, bookmark fragments, and newspaper clippings included, and it's always fun to ponder how they got there. (Who is this person and how did he get the book? Is he the one who scanned it? Etc.)