Yes, you can adjust the size depending on whether you imported the pdf as sheet music or chords.
If sheet music:
First, make sure you use the selection area to eliminate ‘white space’. You can do this by holding down the left button of the mouse and dragging a rectangle. If there is more than one page this area will be applied to all the pages you select. It may be that the original page you imported is small so you need to select a higher DPI value than 100. Experiment with 200 or other settings until it fills the screen. Too large a value will mean that staff lines could be lost.
With a widescreen select 2 or 3 pages per screen if the pages are portrait. If you select half a page at a time with a high DPI you can view the page very large.
The only limitation is the original quality.
Split the text by inserting page breaks (using the page break button). If you have selected more than one page per screen to view as columns then make sure there are no long lines because they will not be wrapped.
I hope this helps. Let us know how you get on.