Download the right format for printing
For anything printed, download the SVG. It's a vector file, so it scales from a business-card sticker to an A3 poster without ever going blurry. The PNG is fine for screens and quick home printing, but SVG is what you hand to a professional printer for a flawless code every time.
Where to place a printed WiFi code
Put it where people look when they want WiFi: table tents, the bottom of a menu, a fridge magnet, a reception card, or a window decal. Keep it at least 2 cm wide, leave a little white space around it (the "quiet zone"), and add a caption like "Scan to join our WiFi" so guests know what it does.