Printing from Gmail doesn't normally involve File Explorer or Opus.
If you print something to a PDF file instead of a printer, you'll get a Save As dialog (which is similar to File Explorer, but isn't File Explorer itself) to choose where to save it to, but that's the only thing I can think of. If you mean that part, Can Opus replace the File Open/Save dialogs other programs use?
Increasing the system DPI or font size settings is worth considering if you're finding File Open or File Explorer windows difficult to read. They should respond to those changes and use larger fonts if told to. (Opus will also respond to them, in addition to letting you choose your font and size directly.)