I would like to add an attachment/PDF to the form, so that when attendees fill it out, they can click to open up a PDF.

I know there is a way to attach a PDF URL (webpage), but why can't I just strictly attach a PDF document?

If I can understand, you are looking for a way to attach a file in your form, this is possible to do and you only need to include the link in the form, for example, you may include either a PDF file or other type of file, you need to upload this and then provide the link in the form, to do this you should add a text field to your form, you will find it under the Form Tools in the Form Builder:

Once it is added then you only need to follow the steps as on the image below:

If you want to embed the PDF file in your form you are also able to do it using our iFrame widget, you will find the widget here: http://widgets.jotform.com/widget/iframe_embed. 

This guide will help you to add the widget to your form: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form. 

Here is also a sample form where you will see the result of using the iFrame widget to embed a PDF file: https://form.jotform.com/60838735487975. 

You will need to upload the file to a server and then paste the link into the widget: 

Hope this helps.