On some of the downloads, they have an "Index" on the side toolbar which can get you to certain places without having to scroll through the whole deal to get there. On others, in the "Page" listing at the top, you can change the first page number (to an estimated value of where you want to be) out of the total pages number. Then things go directly to that page.
Once the download is finished and you open the file, look at the upper toolbar, the one that drops down for .pdf files There's a "Save as" icon near the rh of that toolbar, so you can save the whole thing to Documents, if desired.
Be aware, too, that some of the illustration pages will display "sideways". There's an icon in that second toolbar to rotate the images like that.
Once you figure out how to navigate things, it will be easier to deal with.
Enjoy!
CBODY67