I knew it was coming, and I hit the limit of 300 database fields. 
I am creating a complex pdf invoice with lots of data using the attach PDF using template where template is treated as HTML page.
I have to bring all the data from other databases in as fields into the invoice database as the markdown only refers to fields in the database where the attach PDF rule is executed.
I still short about a 100 fields 
What can I do ?
Wow thats a lot of fields…
I’m a bit confused what you mean. You can set {{Relation.Property}} to reference fields from other databases in markdown. Doesn’t this solve?
Oops, I have missed that one and wasted a lot of time, again hidden in plain sight. I will try it out. 
Ok, I have been reducing the number of fields in my database down to 230 using the suggestion of not having to bring in all the data fields into the database by referencing data in one of my 43 databases in the space, but still get the message that the maximum number of fields must be less than 300.
I should be able to add the 50 fields I short for my complex invoice.
Please Help !! Any suggestions welcome 
Working solution from the Fibery Human,
Hi! Perhaps there are soft-deleted ones that keep you from adding a new field. You can check it here and hard-delete them if you don’t need this data. Just add /audit-soft-deleted-fields to the name of your Workspace, for example:
workspacename.fibery.io/audit-soft-deleted-fields