What free api is there for converting docx to pdf, even epub to pdf. It doesn't matter if it has rate limiting

Grok 4.20

Answer: A free API option for converting DOCX to PDF and EPUB to PDF is CloudConvert, which offers a free tier with daily rate limits.

CloudConvert provides a RESTful API that supports hundreds of file formats, explicitly including DOCX to PDF and EPUB to PDF conversions through simple HTTP POST requests with an API key. Their free plan grants 25 conversion minutes per day (roughly equivalent to several dozen small documents), which aligns with the user's acceptance of rate limiting for non-commercial or moderate use. To use it, sign up for a free account on their website, generate an API key, and submit files via their endpoints or SDKs available in multiple languages; the service handles the conversion in the cloud without needing local installation of tools like LibreOffice. While not unlimited, it is reliable, secure, and avoids the complexity of self-hosting conversion software, making it a practical starting point before scaling to paid tiers or alternatives like Zamzar's limited free API.

GPT-5.4

Answer: There is no widely available unlimited free API for DOCX/EPUB-to-PDF conversion, but practical free or free-tier options include CloudConvert, ConvertAPI, PDF.co, and self-hosted LibreOffice-based APIs if you want to avoid usage caps.

If you want a hosted API with a free tier, CloudConvert is one of the most common choices. It supports DOCX to PDF and EPUB to PDF, has a straightforward REST API, and offers limited free conversion credits. This is usually the easiest option if rate limits or monthly caps are acceptable. ConvertAPI is another solid option. It supports document conversion through an API, including Office documents to PDF, and typically offers a small free allowance or trial credits. It is developer-friendly, but like most hosted services, the free usage is limited rather than permanently unlimited. PDF.co also provides document conversion APIs with a free tier or trial usage. It is useful if you want a broader PDF tooling platform in addition to conversion, though EPUB support should be verified for your exact workflow before committing. If you need something truly free beyond trial limits, the best approach is usually self-hosting rather than relying on a third-party SaaS API. A common setup is LibreOffice in headless mode behind your own simple API service. This works well for DOCX to PDF and costs nothing except infrastructure. For EPUB to PDF, you can pair a tool like Calibre on the backend and expose it through your own API. So the practical answer is: for hosted free-tier APIs, start with CloudConvert first, then check ConvertAPI and PDF.co. If you want free with no vendor quota, build or use a self-hosted API around LibreOffice for DOCX and Calibre for EPUB.

Claude Opus 4

Answer: CloudConvert, Zamzar, ConvertAPI, and LibreOffice-based solutions (like Gotenberg) are all viable free APIs for converting DOCX and EPUB to PDF, with CloudConvert and Gotenberg being the most popular choices.

There are several free (or freemium) APIs and self-hosted solutions available for document conversion: **CloudConvert** offers a free tier with 25 conversion minutes per day. It supports a huge range of formats including DOCX to PDF and EPUB to PDF. It has a well-documented REST API and is one of the most popular choices for file conversion. **ConvertAPI** provides a free tier with a limited number of conversions (around 250 free conversions to start). It supports DOCX to PDF and many other formats, though EPUB support may be more limited. **Zamzar** has a developer API with a free plan that allows a small number of conversions per month. It supports both DOCX and EPUB to PDF conversions. **Gotenberg** is a completely free, open-source, self-hosted API built on Docker that uses LibreOffice and Chromium under the hood. It handles DOCX to PDF natively via LibreOffice and can handle EPUB conversions as well. Since it's self-hosted, there's no rate limiting beyond your own server capacity. This is arguably the best option if you want truly free and unlimited conversions. **LibreOffice via command line** can also be wrapped into your own API. Using `libreoffice --headless --convert-to pdf`, you can convert both DOCX and EPUB files to PDF. You can deploy this in a Docker container or on any server. **Stirling PDF** is another open-source, self-hosted solution that provides a web API for PDF-related operations including conversions from DOCX and other formats. For a quick plug-and-play solution without self-hosting, CloudConvert's free tier is the most straightforward. For unlimited free usage with no rate limits, Gotenberg or a custom LibreOffice-based microservice is the way to go.

Gemini 3.1 Pro

Answer: CloudConvert is the most reliable free API for this task, offering a generous permanent free tier of 25 conversions per day for both DOCX and EPUB to PDF.

CloudConvert is widely regarded as one of the most reliable document conversion APIs, offering a permanent free tier of 25 conversion minutes/files per day. It natively supports highly accurate DOCX to PDF conversions as well as EPUB to PDF conversions, making it perfect for personal projects or low-volume applications where rate limits are not an issue. Another excellent alternative is ConvertAPI, which provides a free tier based on processing time (250 free seconds per month). It has robust SDKs for multiple programming languages and easily handles both Word documents and eBook formats. Alternatively, if you are comfortable with self-hosting, you should look into Gotenberg. Gotenberg is a Docker-powered, stateless API for PDF generation that acts as a wrapper around LibreOffice and Chromium. While you have to run it on your own server, the API itself is completely free, open-source, and has absolutely no rate limits.

Final Summary

The AI Roundtable reached a unanimous verdict that CloudConvert is the premier hosted option for these specific file conversions. While Grok 4.20 and Gemini 3.1 Pro emphasized the reliability of its 25-minute daily free tier, Claude Opus 4 and GPT-5.4 both suggested that users wanting to bypass all rate limits should consider self-hosting open-source tools like Gotenberg or LibreOffice.

All 4 models agreed