For each type of document, from its editing form, you can add attachments in pdf format.

The option is available at the bottom of the document edit page:

The attachment can be prepend and the following image is an example to see how it looks like:

Or append:

There is also the option of adding annexes both, prepend and append in the same document.

Click here to see how you can add a default attachment.

Keep up the good work!