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 “Add Custom Fields”.
Step 3: Select “document” section >> Click on “+ New Custom Field”.
Step 4: Add the “field name” >> Click on the “document type” and select the documents in which you want to show the field >> “Select the type” >> Click on “Save”.