This will be generally caused if the server is 64bit Machine.
To fix this issue, you need to enable 32-Bit Applications in IIS as show in below screen shot.
Check the bullet point 5 here: http://codeutil.wordpress.com/2013/09/16/convert-html-to-pdf/
Fix for Error "Could not load file or assembly 'Pechkin' or one of its dependencies. An attempt was made to load a program with an incorrect format"
This will be generally caused if the server is 64bit Machine.
To fix this issue, you need to enable 32-Bit Applications in IIS as show in below screen shot.
Check the bullet point 5 here: http://codeutil.wordpress.com/2013/09/16/convert-html-to-pdf/