Humble Bundle PDFs are vectors and text-based text, not images. They are also much higher quality that the epub and cbz provided.

But you can use mutool to convert the HQ pdf to HQ CBZ. I use 1920 since that's the height for kindle scribe, but you should do whatever your kindle height is.

Edit: Look at bottom post for faster method.

Video version (windows): https://www.youtube.com/watch?v=5csuFRTDgCU
Video version (macos): https://www.youtube.com/watch?v=L5v6SHWob3g

Windows:

winget install --id=ArtifexSoftware.mutool  -e
for %%I in (*) do mutool convert -b cropbox -O height=2480 -o "%%~nI.cbz" "%%~nI.pdf"

macOS:

brew install mupdf-tools
for f in *.pdf; do mutool convert -b cropbox -O height=2480 -o ${f%%.*}.cbz $f; done

https://mupdf.readthedocs.io/en/latest/mutool-convert.html

This is probably a better idea than adding epub support in

• EPUB Input [Enhancement] #277