Go to the Lead Details page of your lead. Notice the "Upload Document" button (right below the "New Proposal" button):
When you click the "Upload Document" button, you will see a prompt where you can choose a document to upload, and give it a name.
Clicking the "Browse" button will allow you to navigate among the files on your computer and choose one to upload. You can type in a descriptive name, as well.
Once you've uploaded documents for the lead, they will appear in the list, along with the generated proposals.
Notice that a different icon is used to denote an uploaded document (as opposed to a generated proposal); the icon includes an up-facing arrow. Also, the details include the user who uploaded the document, and the original file name.
When you finish a proposal and are ready to email it to your customer, click the Next button on the Preview page or the Email tab found on the top menu.
Under the "Documents to Send" section on the right, the current proposal is automatically pre-selected.
You may select any of the other documents that have already been uploaded for the lead. All selected items will be attached to the email.
You can also upload a new document right from this screen, so that you don't need to go back to the Lead Details page.
One more thing . . . back on the Lead Details page, hovering over an uploaded document will reveal links you can click to act on the document. You can change the display name, download the document, and delete it.