Qarib Haider

This plugin adds a QR Code for each post (link) published on the site. The plugin support all the registered built-in and custom post types, along with pages, this can be changed under the settings page of the plugin.
<h3>Settings</h3>
Following are the available configurable options in the plugin
 Option 
 Default Value
 Size 
 300 pixels
 Margin 
 20 pixels
 Image Type 
 PNG
 Post Types 
 [All Available Post Types]

Link QR Code

Add a QR code to each post, page and custom post type

Arnoud Klaren

Only for registered ScanCircle partners. See the <a href="https://www.scancircle.com/scancircle/partner-program" rel="nofollow ugc">ScanCircle partner program</a>.
Login to your ScanCircle partner page and select the Scan Widget tab to generate the required ScanCircle shortcode and copy/paste it into your post.

ScanCircle

Shortcode handler for the scan widget on ScanCircle partner websites.

Link QR Code vs ScanCircle