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pdftk ... pdf tool kit (extracting and combinining pdf files)
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Useful URLs: 
  https://www.linux.com/training-tutorials/manipulating-pdfs-pdf-toolkit/
  https://www.pdflabs.com/docs/pdftk-cli-examples/

	#combining pdf files
$ pdftk file1.pdf file2.pdf cat output file12.pdf

	#extract range of pages (in this case pages 12-15)
$ pdftk file1.pdf cat 12-15 output outfile.pdf

	#removing selected pages (10-25, in this instance) from a file
$ pdftk file1.pdf cat 1-9 26-end output outfile.pdf

	# ... several files but with selective pages 
$ pdftk A=in1.pdf B=in2.pdf cat A1-10 B2 B5-7 output out.pdf

	#extracting all pages from a file 
$ pdftk input.file burst    #one pdf file per page
	
	#collage scanned pages
$ pdftk A=even.pdf B=odd.pdf shuffle A B output collated.pdf
$ pdftk A=even.pdf B=odd.pdf shuffle A Bend-1 output collated.pdf

$ pdftk in.pdf cat 1east 2-end output out.pdf   #rotate only first page
$ pdftk in.pdf caat 1-endsouth output out.pdf 	#rotate all pages by 180 degrees

	#apply background watermark
	#repair corrupted files