Problem
You have a business requirement to display documents in the page viewer control on a Nintex O365 Form to display runtime 'dynamic' values, meaning reading live inputs from the form as the user is changing the control's values.
The following steps detail how to display the document either by using a hard-coded URL from the document library or by using a dynamic URL.
Solution
The best way to know if a control supports this type of dynamic value reference is to put focus on the property and look at the Insert Reference options. If you have the option to insert Inline Functions this means that the property will only read the dynamic value when the form is first loading. When you see Inline Functions you will NOT see Named Controls.
Another control like the Calculated Control that does support dynamic references will show Runtime Functions and Named Controls, and will NOT show Inline Functions. I have provided screenshots below showing this.Page Viewer Control Settings:The Source URL will be populated with the value of the Title column and not the value from the Title control.Calculated Control SettingsThe Formula will be populated with the value of the Title control and be updated whenever the control's value is updated.