Ad page optimization?
Your getParam code has a bug and cannot possibly work..

function getParam(paramName){                //Look for parameters
  var prmstr = window.location.search.substr(1);
  var prmarr = prmstr.split ("?"); //This has to be ampersand!!
  var params = {};

  for ( var i = 0; i &lt; prmarr.length; i++) {
     var tmparr = prmarr[i].split("=");
     params[tmparr[0]] = tmparr[1];
  }
  return params[paramName];                      //Return the requested parameter
}


This function could be shorter if you skipped a few intermediary and some unnecessary steps:


There is no need to assign anything to an object, and hence to create that object
You only split out prmstr, no need to put that in a var
You could return early out of the for loop


That would give something more like

//Look for parameters
function getParam( s )
{                
  var a = window.location.search.substr(1).split("&amp;"), i = a.length, pair;
  while(i--)
  {
    pair = a[i].split("=");
    if( pair[0] == s )
      return pair[1];
  }
}


or, more Golfic

function getParam( s )
{                
  return location.search.substr(1).split(s+"=")[1].split("&amp;")[0]
}


Then, someone clearly copy pasted 3 times the same piece of code and made minor changes.
You should have 1 generic function, called 3 time instead. Or, realize that this code is a bad replacement of unescape and just call that. The reason the replacement is bad is that there are lot more characters out there than just %20.

var keyword = unescape( getParam( 'keyword' ) ),
    file_name = unescape( getParam( 'file_name' ) ), 
    file_url = unescape( getParam( 'file_url' ) );


Furthermore, you assign google_ad_client = "ca-pub-1582371570610820"; and google_ad_slot = "9662168393"; twice.

Be aware , document.write, I have been told by reliable sources, should only be used when standing inside a protective summoning circle, lest greater demons devour you. Instead you should have the link tag in your HTML and then find the link in your js and fill in the anchor and caption.

Finally, this is not very good code, I hope the server side code that downloads the file is of higher quality and will not allow path manipulation or path traversal.