PDF.co uses a credit-based system. The credits refer to a system-assigned number that the engine consumes to process an API request.
For example, to Add Text and Images to a PDF document, the PDF.co API will require 21 credits to process the request successfully.
The API functions/modules have their own predefined number of credits. These credits are spent per document page. When there's no document page involved such as Credit Balance Check, the credits will be taken per call.
You can find the list of functions and their corresponding credits here https://app.pdf.co/subscriptions/credits_pricing