Fit current page to width
Clicking "Fit Width" (W) sets the zoom level such that the widest page of a PDF is fit to width.
In most textbooks, there is a page (or a few of them) that are wider than the rest of the pages (or are in landscape orientation). "Fit Width" fits these pages to width, so all the other pages are thinner than the screen width, causing text to be smaller. So, the "Fit Width" option should instead fit the current page or the most common pages to width. (Pinch to zoom is completely broken, and Ctrl + (two-finger) scroll requires two hands.) If "Continuous" is disabled (i.e., C is pressed), then only one page is in view at a time so it is fit to width. However, scrolling does not work then. Adobe Acrobat Reader can fit current page to width (by resizing wider/thinner pages).