Sprout Studio doesn't explicitly allow you to export as PDF, but most modern operating systems and browsers offer a "Print to PDF" functionality.
Since all client-facing pages in Sprout Studio (contracts, invoices, questionnaires, etc.) are optimized for print, the best way to get a PDF of any of those pages is to use the built-in "Print to PDF" functionality.
To export or download PDF of a contract, invoice, questionnaire, or any other page in Sprout Studio, just click File > Print and then choose the "Save as PDF" option.