$ pdftopng
pdftopng version 3.04
Copyright 1996-2014 Glyph & Cog, LLC
Usage: pdftopng [options] <PDF-file> <PNG-root>
  -f <int>          : first page to print
  -l <int>          : last page to print
  -r <int>          : resolution, in DPI (default is 150)（分辨率默认 150 dpi）
  -mono             : generate a monochrome PBM file（二值化）
  -gray             : generate a grayscale PGM file（灰度图）
  -freetype <string>: enable FreeType font rasterizer: yes, no
  -aa <string>      : enable font anti-aliasing: yes, no
  -aaVector <string>: enable vector anti-aliasing: yes, no
  -opw <string>     : owner password (for encrypted files)
  -upw <string>     : user password (for encrypted files)
  -q                : don't print any messages or errors
  -cfg <string>     : configuration file to use in place of .xpdfrc
  -v                : print copyright and version info
  -h                : print usage information
  -help             : print usage information
  --help            : print usage information
  -?                : print usage information

$ pdftopng demo.pdf -r 300 -mono ./demo-r300-mono/demo