Filling a third party PDF form with Salesforce data

I have been trying to figure out how to fill a PDF form with contact data and I found a helpful post here. I followed the instructions but it didn't say what to do with the code or how to set up the button. I tried putting the first two blocks of code in an apex class and then the third as a Visualforce page. I then tried to add a button on the contact to pull up that page but it doesn't let me choose the page.

Here is my Apex Class.

public class myXFDFController {

Private String getXmlString(Contact c) {
    String s = '<?xml version="1.0" encoding="UTF-8"?>' +
        '<xfdf xmlns="http://ns.adobe.com/xfdf/" xml:space="preserve">' +
        '<f href="https://www.riverblufftech.com/wp-content/uploads/2015/11/myForm.pdf"/>' +
        '<fields>' +
        '<field name="First Name, Middle Name, Last Name & Suffix"><value>' + c.firstname + ' ' + c.lastname + '</value></field>' +
        '<field name="Email address"><value>' + c.Email + '</value></field>' +
        '</fields><ids original="E1D407DB3A404A389E73EFCE11A5CE0B" modified="46721BE9257A47A3A1F3CCEE45E9F19A"/>' +
        '</xfdf>'; 
    return s;
}

public PageReference XFDFInit() {
    Contact c = [SELECT Id, firstname, lastname, email FROM Contact WHERE id = :ApexPages.currentPage().getParameters().get('id')];
    String xmlContent = getXmlString(c);
    Attachment attachment = new Attachment();
        attachment.Body = Blob.valueOf(xmlContent);
        attachment.Name = c.lastname + c.firstname + '.XFDF';
        attachment.ParentId = c.Id;
        insert attachment;

    PageReference contactPage = new PageReference('/' + c.id);
    contactPage.setRedirect(true);
    return contactPage;
}}

And this is my Visualforce Page, I feel like there should be more here but the post I linked to earlier seemed to say you only need this.

<apex:page showHeader="true" controller="myXFDFController" action="{!XFDFInit}"></apex:page>