This article describes how to make fields mandatory.
Step 1. Open the template and add text field
Step 2. Choose text field → Properties tab → Required (marked with red) set to Yes
Step 3. Deploy the template. Click Publish button (marked with red) on the right bottom corner
Step 3.1 Publish PDF form window appears
New form can be access by generated link or by navigating to the target library.
Step 4. Open a new form. Mandatory field has red border around it
Try submitting the form (click Submit button) with empty mandatory field. Following warning prompt appears
Fill in the field and form will be submitted.