I do have instructions for saving pdfs, but they're specifically for files with text and shapes in them.
Files like yours should only ever be saved as jpegs.
I'm sorry you've got such a stupid printer
I expect you'll find the big files are 16-bit instead of 8-bit.
If your file had any text or vector shapes in it, it would be correct not to flatten.
But since it's just an image, you definitely must flatten before saving.