I came across that I have to fix a lot of images, and sometimes multiple of PDF files each has only one page.
Tools here are in Ubuntu Linux; in Windows, it should be accessible via WSL; not sure about macOS.
sudo apt install imagemagick poppler-utils
Then you now have two important commands -
Now, I have another important problem - ImageMagick security policy 'PDF' blocking conversion
Though, my solution is
sudo nano /etc/ImageMagick-6/policy.xml
To issue commands, for some reasons,
convert supports wildcards, but not
convert "$FOLDER_NAME/*.GIF" "$FOLDER_NAME.pdf" pdfunite x/x1.pdf x/x2.pdf x/x3.pdf x.pdf
Lastly, this post is possible because of