Incoming requests are rate limited to prevent a single user from hogging the resources. By default PDF-ace.com will cache the generated PDF for 24 hours and your visitors can request the cached PDF file as many times as they want. But if you choose to disable the cache and generate a new PDF for each request, the request is limited to one request every 20 seconds. This request rate is subject to change.
Note: This applies only to "Save as PDF" buttons and is not relevant to the HTML to PDF API.