Ffmpeg and imagemagick.

magick -density 150 source.pdf page-%03d.png && ffmpeg -framerate 1/5 -i page-%03d.png -vf "scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2" -c:v libx264 -r 30 -pix_fmt yuv420p slideshow.mp4

For example, I did not run this.