You can try to create an onclick javescript button and use something like the followign to check for null values and then redirect to your VF page where you are rendering as a PDF.  First you will setup your variables and then check to see if they are empty/null and then display the message you would like and then redirect to your Vf page. 

If this helps you out, please mark as best answer...

Shawn

var naics = "{!Lead.NAICS_Code__c}"; 
var sltn = "{!Lead.Solution__c}"; 
var cmpny = "{!Lead.Company_Channel__c}"; 



if(naics == ''){ 
html = '<div id="dialog" style="display: none" title="Error"><p> INSERT ERROR MESSAGE HERE {!$Profile.Name} </p></div>'; 
dialogBox(html); 
}else if(sltn == ''){ 
html = '<div id="dialog" style="display: none" title="Error"><p>INSERT ERROR MESSAGE HERE </p></div>'; 
dialogBox(html); 
}else if(cmpny == ''){
html = '<div id="dialog" style="display: none" title="Error"><p> INSERT ERROR MESSAGE HERE </p></div>'; 
dialogBox(html); 
}else{
window.location = '/INSERT VF Page HERE.jsp?retURL=%2F{!Lead.Id}&id={!Lead.Id}'; 
}

function dialogBox(){
{!REQUIRESCRIPT('//ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js')} 
{!REQUIRESCRIPT('//ajax.googleapis.com/ajax/libs/jqueryui/1.11.2/jquery-ui.min.js')} 

var $g = jQuery.noConflict(); 
$g(function() { 
$g('head').append('<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">'); 
$g('body').append(html); 

$g("#dialog").dialog({
dialogClass: "no-close", 
autoOpen: true,
buttons: [
{
text: "OK",
click: function() {
$g( this ).dialog( "close" );
}
}
] 

}); 
});
}