combine PDFs

This will usually give MUCH lower filesizes than using imagemagick. Using imagemagick will effectively convert each page to an image. Ghostscript instead merges them properly, BUT you can run into font issues if you don't locally have the fonts used in the source files:
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combine.pdf -dBATCH 1.pdf 2.pdf 3.pdf 4.pdf 5.pdf

PDF to txt

This will grab the text from a PDF and print it to STDOUT. It wont use OCR or anything fancy if the PDF is just images:
gs -dBATCH -dNOPAUSE -sDEVICE=txtwrite -sOutputFile=- letter.pdf

remove first or last pages in a pdf

The dLastpage=10 option or dFirstPage=3 option allow specifying a start and end page when processing a pdf.