The booking form and confirmation emails do not support attachments or uploading of documents. However you can add a link to your own online storage site (ie Dropbox, Google drive) so your bookers can download needed files.
Link to a file on your booking form
Under booking form Questions, click + Add a new question.
Select type Block of Text. In the label field use Markdown to hyperlink to the site that is hosting your attachment.
Add link in Confirmation Emails
Navigate to Notifications Actions After New Booking made. In the Confirmation email to the booker, edit the email content by clicking on the link icon in the toolbar to hyperlink to where your booker should download the file.
The same steps can be repeated in any email that sends to your booker.