What is the best way to generate email messages with attachments from Twig files? I am coming from a .net/C# background and we have recently moved to a Craft/Twig platform.  What is the best way to do this: Have a visitor come to a Craft webpage (Twig file), enter their email address in a text box, hit a button, and have an email message sent to them containing a PDF attachment that they selected from that page?

Is this functionality built-in to Craft or do I need a plugin?
You can do this with a simple Craft plugin.  You can generate the boilerplate plugin code here.

Your form on the front-end will set its hidden action input to your plugin's controller's action.

From there, you can use Craft's EmailModel (along with addAttachment()) and EmailService's sendEmail() to send whatever email contents you wish.