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...when I imported the .PDF file, Gimp brought it in as a 100dpi image, which yields microscopic images instead of a full page. The full deal is that I want to import 8.5x11 inch PDFs, fiddle them, and then print 'em out -- full scale, please.
What is the "standard" work flow for doing that?....
Gimp will always open with a default 100 pixels-per-inch (ppi) Up to the user to set another value. Usually for quality printing that is 300 ppi but it also depends on how the PDF was originally created. Using Okular? then look in file -> properties for information.
Once open in Gimp, any vector properties are gone for good, so do not start scaling up and down to fit, all that does is degrade the image. Get the PDF into Gimp at best size for printing to your paper size (USletter you said) Then do any editing before printing.
...so many if'-and-maybe's a little 3 minute video demo. One way to start off.
https://youtu.be/B8O4v-0HK9M