I use PDFBox. There are some FOSS layout libraries you probably want to add one or more of, depending on your needs.

It's disgustingly fast and capable. In one project we crunched out 150k PDF documents in less than forty minutes from roughly 6 GB input data, on a mid laptop, including a fair bit of other file types related to those documents.

Fairly low level but not hard to get started with. You might have to wrap it in a module yourself if you're using those.