How can I convert 100 photos into a pdf thats less than 6 MB?
You can use ImageMagick (free, open source, cross-platform, CLI) with the jpeg:extent=value parameter to compress each image to a target size, e.g. mogrify -define jpeg:extent=60KB *.jpeg

Restrict the maximum JPEG file size, for example -define jpeg:extent=400KB. The JPEG encoder will search for the highest compression quality level that results in an output file that does not exceed the value. The -quality option is ignored if it is also present.

Then see How can I convert JPG into PDF easily?