Convert PDF to PNG using Linux

If you use the defaults for Imagemagick, the output of a PDF file with text converted to PNG is unreadable.  The secret is to use the density option:

$ convert -density 300 xxxxx.pdf yyyyy.png

If the original PDF has a transparent background, use the alpha option:

$ convert -density 300 xxxxx.pdf -alpha off yyyyy.png

[update 2018]
can also use

$ pdftoppm input.pdf outputname -png