Merging PDF, careful to not omit last argument:

pdfunite in-1.pdf in-2.pdf in-n.pdf out.pdf

Multiple pages per sheet

pdfjam --a4paper --nup 2x1 in.pdf --outfile out.pdf --landscape

Create empty page

convert xc:none -page A4 portrait.a4.pdf
convert xc:none -page 842x595 landscape.a4.pdf

Extract pages

qpdf thesis.pdf --pages . 31-38 -- extract.pdf

Extract all images from a document (pdfimages)

mkdir out
pdfimages -all document.pdf out