PDF manipulation
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