I think you training Gmail that the email shouldn’t go into Promotions will help teach all of Gmail.
More people need to do it for Gmail to start getting the hint.
Also, might need to reach out to PandaDoc and let them know what’s happening so they can investigate on their side.
Since their service is sending the email, perhaps they can change things about the email that will help prevent it from landing in Promotions.
Lastly, you could always use your own email service to send the email to the client.
These types of “manual” emails are more likely to end up in an Inbox.
The more custom it is, the better.
For example, if the emails contains details specific to that client rather than a generic message all clients will get, such as “Hello, you have a new doc, please click here to open”.