The HTML that JSONCV generates is optimized for printing in an A4 letter, so it can be used to create a well-formatted PDF document.

You can first click "Download HTML", then open the HTML in a browser and choose "Print" from the menu (or press ⌘+P), it will open a modal where you should find the option to save it as a PDF.

For best compatibility, I recommend using Firefox rather than Chrome to complete this process.