Quote:..It increased the resolution but did nothing to feather the lines. It was supposed to blend the selected foreground with the white background. Didn't seem to work.
It is as Ofnuts wrote. Really you should start off in a suitable format.
Changing the resolution - pixels per inch / ppi / aka dpi - that only changes the print size, the important bit, the image size is not changed.
All in one screenshots:
https://i.imgur.com/dxXFnmH.jpg
1. Scale the image,
Image -> Scale Set Width/Height to 400% then X/Y resolution to
288 ppi 384 ppi (edit - not paying attention, I thought you were using original 72 ppi) and interpolation NoHalo or Cubic
2. That gets a larger image size with a little bit of antialiasing.
3. If that is not enough, apply a little bit of blur
Filters -> Blur -> Gaussian Blur
4. Then use the curves tool
Colors -> Curves to (hopefully) improve.
Quote:The drawing has to be in PDF/A - I heard that GIMP exports to PDF
I think that spec. is often used with CMYK printing. You need to ask the printing company (usually a PITA)
Gimp will export a vanilla PDF in bitmap (raster) form ie. Text is flattened to an image. If you need to set up a mixed page of image + text and PDF/A you need Scribus.
https://www.scribus.net/
Otherwise
Try LibreOffice Draw. Part of
https://www.libreoffice.org/ Set up a user page size, import the scaled up image & set to size. Save as a PDF/A, as screenshot:
https://i.imgur.com/NjGg106.jpg