Woukld be helpful if we knew what the exact error message was? But based on your description it sounds like you are not forming the path to your file properly and your code certainly isn't 100% reliable. I would say you need to do the following to get a well-formed, proper file address:
String _fileNam = System.IO.Path.Combine(AppPath, @"\master\Voucher.pdf");
If this doesn't solve your issue then check the value of
_fileNam
by debugging - if you can't really debug, get your code to send you an email containing the path of the file (but don't try attaching the file to that email.) Then, take that path and paste it into a windows explorer (NOT internet explorer) address bar on your server - if it works, you have a different issue to the file not being found. Also, make absolutely sure you have copied all the files to your web server.
Hope this helps,
Ed