I’m a classical musician and obtain many of my pieces in .pdf from IMSLP, but they can be small and difficult to read when I go full screen. I can zoom in and make them more legible, but when I move to the next page the music reverts to the original size. That makes for bumpy performances. Is there any way to zoom and keep the zoom from page to page?
The music should automatically expand to fit the screen, so perhaps there is a lot of white space surrounding the music? When you import a PDF you can select the area of the page to import, so if there is a lot of white space surrounding the music just select the music area. If the quality is reduced try increasing the DPI when importing.
You can also crop existing pages of PDFs already imported by going to the sheet music editor. Select the page, drag with the mouse to select an area then press Crop. Save the song and see if that helps.