Merge pdf file in Ubuntu
alternative:
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combinedpdf.pdf -dBATCH 1.pdf 2.pdf 3.pdf
extract images from pdf
pdftk *pdf cat output ../FINAL_NAME.pdf
Lot of other tricks with pdf including OCR(not working very well, It would need some boosting - language model, domain adaptation, contrast adjustment, .., etc) in tutorial: http://blog.konradvoelkel.de/2010/01/linux-ocr-and-pdf-problem-solved/
pdfimages -j Arco\ Big\ Walls-en.pdf arco-big