Instead of low-quality screen-shots a PDF to get the images, use Poppler to extract the original high-resolution images from the PDF. Note: only raster images can be exported with Poppler.

Examples of PDF image extraction tasks:

List all PDF images:

pdfimages -list in.pdf

Extract PDF images from all pages, dumping all images in mydoc.pdf to the same directory. Filenames start with out-. There might be a lot of images.

pdfimages -all in.pdf out

Extract PDF images from specific pages: example is for page 3 only:

pdfimages -all -f 3 -l 3 in.pdf out

-f

first page to extract

-l

last page to extract

Install Poppler:

• Linux: apt install poppler-utils
• macOS / Linux (Homebrew): brew install poppler
• Windows: use WSL poppler

Related: extracting a page(s) from PDF