HowTo: Convert, resize and center PNG into PDF

convert puma1.png -gravity center -background white -compress jpeg \
  -resize 1755x2475 -extent 1755x2475 -units PixelsPerInch \
  -density 150x150 puma1.pdf

HowTo: Join multiple PNG files together

a) Horizontal join:

convert ifile1.png ifile2.png +append ofile3.png

b) Vertical join:

convert ifile1.png ifile2.png -append ofile3.png