Create a PDF from an image

Until now I was using the Page Scaling feature of acrobat reader to print PDF file with pages of random size. I’am creating such PDF with Imagemagick:
convert myimage.png myimage.pdf
The drawbacks of this dirty methods are:

• All pages are extend to fit the full page, which is not always what you wants
• You may want to print from a tool which does not have the Page Scaling feature.

There is a solution in the Imagemagick documentation:
convert -units PixelsPerInch -density 150x150 myimage.png mypdf.pdf