Use  case:
I am using  Guest User license and they are assigned as an app user in Salesforce however Salesforce restricts me to be able to configure any permissions for that user. I need for Guest User license to be able to trigger process builder that will autosend an email to contact email shown on record but it is not working for this user though is working for another user than is system admin.

Since RS Documents app is configured to use contact email from Salesforce, you need to enable read for contacts setting:

Setup > Sites > Delivery Note > Public Access Settings > Object Settings > Enable Read for Contacts
– Then create sharing rule that will make this Contact visible for Site User.