The Files tab under any trip is capable of storing PDFs, documents, and images related to the trip. These stored files are private to you by default. You can also send a copy to the client.
To save a file, open the trip's Files tab and simply drag and drop into the marked area. Alternately, click inside the dashed line area to open a file select box:
Once you've saved a file here, it will appear in the list. If you'd like to send this to your client, click the three buttons at the top-right corner and click Send:
From there, write the accompanying email, then click Send. Your client will receive the email with this file as an attachment.