I don't have MS Office installed so it's pretty annoying to have to upload the file to Google Drive every time I need to open one.
Is it possible to set up Google Drive to open my
.pptx, etc., files by default, as for Google Docs files?
Download and install Google Drive for PC/MAC. Your drive files will sync to your desktop where those program defaults will supersede. I.E.
xlxs files will open in Excel,
docx will open in Word etc.