There are number of online services to generate QR code images for blog post URLs. Do you want to explore a way to display QR code image on every single post URL without using any plugin?
QrServer QR code generation service make things super easy, quick and completly automated. It allows you to display QR code image based on URL parameter. Just add following code to single post page within WordPress loop to show QR code image for that post URL.
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Above you can customize size of QR code image by changing 500X500 values (make sure width and height are same when you modify QR code size values). You can place the code at the start or end of single post for better visibiliy. Credit Mobi Freaks