Hi,
1. The PDF Plugin seems not indicated in your case, it's to attach the invoice not any pdf files. Moreover it's not included in the business edition.
But when editing the emails you sqhould be able to find the option to attach a file under the tab "attachment". And here you can attach as many pdf files as you want. If you need specific cases, so you will have to create your own plugin.
2. You can create a custom field with the type checkbox, required, and in the text field to describe the checkbox put a text in uppercase like, "WIDERRUF" then via a translation override, transform it as a link (or something else).
Translation like: WIDERRUF="<a class='modal' href='LINK_TOHOME_PAGE'>YOUR_TEXT</a>"
This way, the customer can click on the link to read the widerruf in a popup and will have to check the checkbox to tell that he has read the widerruf. Hope this will help you.