Hi Niki,

Please have a look to the below code and make the changes as per the requirements

-- JavaScript --

{!requireScript("/soap/ajax/16.0/connection.js")}
{!requireScript("/soap/ajax/16.0/apex.js")}
var a = sforce.apex.execute("SendEmail","emailPdf",{localId:"{!Application__c.Id}"});

-- Apex Class Method --

public void SendEmail(Id localId){
 Application__c inv = [Select Id, name From Application__c Where Id=:localId];
 PageReference pdf = Page.PDFpage;// Replace PdfOfInvoice with your Page which render as PDF.
 pdf.getParameters().put('id', localId);
// Blob b = pdf.getContentAsPDF();
 Blob b;
 if (Test.IsRunningTest()){b=Blob.valueOf('UNIT.TEST');}else{b = pdf.getContentAsPDF();}
 // Create Attachment Object to attach with Email
 Messaging.EmailFileAttachment efa = new Messaging.EmailFileAttachment();
 efa.setFileName(inv.Name+'.pdf');
 efa.setBody(b);
 // Define the email
 Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
 // Sets the paramaters of the email
 email.setSubject('PDF of Invoice - '+inv.Name);
 email.setToAddresses( new List<String>{inv.Email__c} );
 //email.setbccAddresses( new List<String>{'admin@gmail.com'} );
 email.sethtmlBody('Hi '+inv.Name+',<br/><br/> '
                        +'Please find the attached Invoice.'
                        +'<br/><br/>'+'Thanks,'+'<br/>'
                        +UserInfo.getName()+'<br/>'
                        +UserInfo.getOrganizationName());
 email.setFileAttachments(new Messaging.EmailFileAttachment[] {efa});

 try{
      Messaging.SendEmailResult [] result = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
 }catch(System.Exception e){
     
 }
 
I hope you find the above solution helpful. If it does, please mark as Best Answer to help others too.

Thanks,
Ajay Dubedi