combine PDFs

The default pixel density is crazy low and all the files wind up looking blurry. This method converts the PDFs to images and stiches them together. As a result the filesize is massively bloated, you can avoid this by using ghostscript instead, but it sometimes has issues with weird fonts:
convert -set units PixelsPerInch -density 200 file1.pdf file2.jpg file3.pdf output.pdf

resize image

This will scale the image down such that none of the sides are longer than 3000 pixels. If it's already smaller than that, nothing will happen. magick big.jpg -scale "3000x3000>" small.jpg

I often combine this with jpegoptim to get small images for sharing.

convert svg to png

It works, but the result kinda sucks. Use inkscape's cli instead.