Linearised PDF files are defined in order and can be read from the start.

https://blog.idrsolutions.com/2010/02/linearized-pdf-files/

I think that's what the "optimize for Web" checkbox does

Wow so it actually does something! I wish programs would use such vague descriptions. (Or more of them had helpful instant tooltips.)

Linearized (a.k.a. web optimized) PDF files only help for displaying the first page quickly. The rest of the file is still in pretty much random access order.

For what it's worth, there's a format for content like pdfs that's optimized for viewing on screens (as opposed to printing), djvu: https://en.m.wikipedia.org/wiki/DjVu

Oh, wow initial release 1998,now I'm feeling a bit old...