You can use ImageMagick to convert multiple PNGs into a PDF and convert a multiple pages PDF into multiple PNGs.

PDF to PNG

$ convert document.pdf document.png

For a document with 10 pages, this will generate document-0.png, document-1.png, … document-9.png.

PNG to PDF

$ convert document*.png document.pdf

This convert the PNG pages into pages in the document.pdf document.