Add PDF handling section to agreements

Author: fornwall

swagger.yml

@@ -488,6 +488,10 @@ info:
 
     ![Agreement Typical Flow](https://www.lucidchart.com/publicSegments/view/30bd1625-75d2-4ece-bb1d-5e3ccee93010/image.png)
 
+    ## PDF handling
+
+    See the [Agreements PDF handling](https://docs.google.com/document/d/1a_HFN-9gtB16RyXm-VVDwMhgJfnh-kZCj8G3QwzzIVc) for recommendations and requirements about PDF agreement documents.
+
     # Partners integration
 
     Partners integration allows a *company recipient* to open up its mailbox to a third party (a *partner*) to allow it to access and process the mailbox content. This is typically used by companies to allow bookkeeping and administrative third parties to access and process invoices and documents.