To add document custom fields only for specific documents, please follow the below mentioned steps:
Step 1: Go to “Document Settings”.
Step 2: Click on “Manage Custom Headers”.
Step 3: Click on “Add New”.
Step 4: Add “Field name” >> Select “Field Type” >> Add any “default value” >> “Select desired Document” >> “Enable toggle button” to show field name on invoice >> Click on “Save”.