You need to specify atleast one input file, and exactly one output file
Use - for stdin or stdout

Name:
  wkhtmltopdf 0.9.0, using wkhtmltopdf patched qt.

Synopsis:
  wkhtmltopdf [OPTIONS]... <input file> [More input files] <output file>
  
Description:
  Converts one or more HTML pages into a PDF document.

General Options:
  -h, --help                          Display help
  -q, --quiet                         Be less verbose
  -V, --version                       Output version information an exit
      --extended-help                 Display more extensive help, detailing                                      less common command switches
      --collate                       Collate when printing multiple copies
      --copies <number>               Number of copies to print into the pdf                                      file (default 1)
  -O, --orientation <orientation>     Set orientation to Landscape or Portrait
  -s, --page-size <size>              Set paper size to: A4, Letter, etc.
  -p, --proxy <proxy>                 Use a proxy
      --username <username>           HTTP Authentication username
      --password <password>           HTTP Authentication password
      --custom-header <name> <value>  Set an additional HTTP header (repeatable)
  -b, --book                          Set the options one would usually set when                                      printing a book
      --cover <url>                   Use html document as cover. It will be                                      inserted before the toc with no headers                                      and footers
  -H, --default-header                Add a default header, with the name of the                                      page to the left, and the page number to                                      the right, this is short for:                                      --header-left='[webpage]'                                      --header-right='[page]/[toPage]' --top 2cm                                      --header-line
  -t, --toc                           Insert a table of content in the beginning                                      of the document
Contact:
  If you experience bugs or want to request new features please visit 
  <http://code.google.com/p/wkhtmltopdf/issues/list>, if you have any problems
  or comments please feel free to contact me: see 
  <http://www.madalgo.au.dk/~jakobt/#about>