convert pdf

note: you need to install imagemagick and ghostscript first

convert multiple png to one pdf

convert page1.png page2.png output.pdf

or simply

convert page*.png output.pdf 

src: http://stackoverflow.com/a/8955465

hint: 
lets say you'd like your oldest file (page01.png) to be the last page in output.pdf.
you can reverse the order of files:

convert page* -reverse output.pdf

reduce filesize of pdf

...via ghostscript

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -sOutputFile=output.pdf input.pdf

src: http://www.virtualzone.de/2012/11/how-to-reduce-pdf-file-size-in-linux.html

fyi: you can change /screen to /printer for better quality

join multiple pdf files into one file

 ...via ghostscript

gs -q -sPAPERSIZE=letter -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=out.pdf in1.pdf in2.pdf in3.pdf ...

...via pdftk

pdftk file1.pdf file2.pdf cat output -

...via pdfsam

pdfsam-console -f 1.pdf -f 2.pdf -o ./output.pdf concat

reverse page order

pdftk in.pdf cat end-1 output out.pdf