The ability to generate downloadable PDFs for Onesheets is available to Professional and Enterprise users. To upgrade, visit here.
To generate a PDF for a Onesheet you've created, simply navigate to your Onesheet dashboard and select the three vertical dots next to the Onesheet you want to download. There, you will see an option to "Generate PDF." Once the PDF is generated (it can take a few minutes), you have the option to download the file to your computer.