I'm unable to center a converted *.png file with ImageMagick's gravity option. Gravity seems to have no effect in the following command. On the contrary it erases everything and the output becomes white page.
convert puma1.png -compress jpeg \ -resize 1755x2475 \ -units PixelsPerInch \ -density 150x150 \ -page A3 \ -gravity center puma1.pdf
ImageMagick version is 6.8.6-9 2014-03-06 Q16.