Quote:At that point it seems I might as well have them print the jpg file, or is there still an advantage to using pdf?
I agree, no advantage in your case, for using a PDF. Remember the D(ocument) in PDF. It is a format best suited to text or mixed text and some graphics. Text in the PDF is a vector format and when the font metrics are included in the PDF the document is viewed or printed correctly. Which is why printers like them.
When it comes to Gimp (or ImageMagick) your bitmap image file is just "wrapped" in a PDF and quality depends on the image. Viewing a PDF, depending on zoom value, will result in on-the-fly scaling and your image will look different to the original.
Your quality problem is you are coming from a low base. 72 ppi which when Print Size is adjusted (42" -> 24") ends up as about 120 ppi. No scaling is used for that, quality is what you originally have, anything else requires scaling, interpolation and degradation.
For regular viewing at a distance 120 might be ok see: http://resources.printhandbook.com/pages...inting.php
If you want it for close inspection and 300 ppi then you are scaling the image 2.5 times and you might get away with that, 600 ppi = x5 and expect fuzziness.
Then it depends on the printing company. This from the one I use.
All prints larger than 18″x 12″ are printed on our Chromira printer and these should be supplied at the required print size at 300 ppi. (For good quality we advise at least 200ppi.) and follows up with, use a jpeg with 95-98 quality or an uncompressed tiff.
That is a photographic laser printer which has a high dots-per-inch (dpi, not the same as image ppi)
Best guess the printing company gives not so specific advice.
If you use the procedure outlined in the video, anything you plant on the blank canvas uses the canvas ppi, 200 / 300 ... as you set up. Export as a (95) jpeg and hope the file size is not too large for acceptance.
Edit
I see from your other posts "I'm using a 4k TV as my monitor" Strictly, for accurate results your Monitor should be color-calibrated using a calibration device. No one expects you to do this for a one-off but you should be aware of:
What you see is almost certainly brighter than when printed. I am not sure this will work with a tv but there is a simple brightness adjuster here: https://www.cambridgeincolour.com/tutori...ration.htm
If printed using an inkjet printer, then because of the cyan-magenta-yellow inks some brighter colours become muted (bright green for example)