To customize pdf in tableau, you need to follow these steps:
By going to File->Print as PDF?
In this you can add title and caption to a sheet and its export to pdf by selecting the option in worksheet-> show title/show caption.
To create a dashboard. Drag your sheet into it and then you can add text/images/breaks to the canvas. This will then be included in the pdf when you go to File-Print to PDF.
Want to be an expert in Tableau? Check out the Tableau Training to get the right assistance.