PrintSocket
Send PDFs, ZPL labels and ESC/POS receipts to your own printers from any language; free tier
HTML to PDF conversion API built on Chromium.
Renderly API is a free Documents & Productivity API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
The developer-friendly PDF generation API. HTML to PDF in one API call.
Details from renderlyapi.com.

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Send PDFs, ZPL labels and ESC/POS receipts to your own printers from any language; free tier
The Renderly API is provided by Renderly (renderlyapi.com). The developer-friendly PDF generation API. HTML to PDF in one API call.
Yes. The Renderly API is listed in the Documents & Productivity category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Renderly API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Renderly API supports CORS, so front-end JavaScript can call it directly.
Yes. The Renderly API is served over HTTPS, so it's safe to use in production.