You can do this in following way:
Before going on the pdfURL page go to a temproray visualforce page and have javascript/controller method to update the record with Number of Views +1. To go on the temp VF Page you can write a trigger as below:
 

trigger AccountURLUpdate on Account (before insert, before update) {
    For(Account a: Trigger.New){
        if(a.Url__c != NULL && a.Url__c !='' ){
            string add= a.url__c;
            boolean containsURL = add.containsIgnoreCase('tempVFPage');
            if(!containsURL )
            	a.Url__c = 'https://ap1.salesforce.com/apex/tempVFPage?url='+a.Url__c+'&recordid='+a.id;
        }
    }
}

And then the code for the tempVFPage can query the record check if it has any number of views and if yes then increase that with 1, the code of this tempVFPage could be something like :
 

<apex:page >
    <script type="text/javascript">__sfdcSessionId = '{!$Api.Session_Id}';</script>
    <script src="/soap/ajax/29.0/connection.js" type="text/javascript"></script>
    <script src="/soap/ajax/29.0/connection.js" type="text/javascript"></script>
    <script>
       	var recordid= getParameterByName('recordid');
    	var myquery = "SELECT Id, No_of_Views__c,Url__c FROM Account WHERE Id = '"+ recordid + "' limit 1"; 
    	var result = sforce.connection.query(myquery);
    	var records = result.getArray("records");
    	if(records[0]!= null){
        	var acc = records[0];
            if(acc.No_of_Views__c !=null)
            	acc.No_of_Views__c = parseInt(acc.No_of_Views__c) + 1;
            else
                acc.No_of_Views__c = 1;
            var updateResult = sforce.connection.update([acc]);
            parent.window.location.href =""+getParameterByName('url')+"";
            
        }
    
    function getParameterByName(name) {
        var match = RegExp('[?&]' + name + '=([^&]*)').exec(window.location.search);
        return match && decodeURIComponent(match[1].replace(/\+/g, ' '));
	}
    </script>
</apex:page>