When I open a PDF file, the file is scaled so that I can see a full page as large as possible (usually setting the page to be as tall as my viewing window but somewhat narrower). This is probably the best option when scrolling is set to horizontal.

However, it is not the correct default behavior for vertical scrolling; in that case, a better default would be for the page to be scaled to occupy the full width of the window, regardless of the page's height.