To export a flow as a PDF, go to the Boards panel on the upper left side and hover over the board you'd like to export. Then, click the revealed "..." options button and select the "Export board as PDF" option to download it.
☝️ On the older versions of the Mac app before the Summer Breeze release, this option is located at the bottom left of your flow canvas.
💁♀️ When a flow is exported as a PDF, the comments in the flow canvas will not be visible in the PDF.