This is quite a hard task, ever harder if you want perfect results (impossible without using Word) as such the number of APIs that just do it all for you in pure Java and are open source is zero I believe (Update: I am wrong, see below).
Your basic options are as follows:
- Using JNI/a C# web service/etc script MS Office (only option for 100% perfect results)
- Using the available APIs script Open Office (90+% perfect)
- Use Apache POI & iText (very large job, will never be perfect).
Update – 2016-02-11
Here is a cut down copy of my blog post on this subject which outlines existing products that support Word-to-PDF in Java.
Converting Microsoft Office (Word, Excel) documents to PDFs in Java
Three products that I know of can render Office documents:
yeokm1/docs-to-pdf-converter
Irregularly maintained, Pure Java, Open Source
Ties together a number of libraries to perform the conversion.
xdocreport
Actively developed, Pure Java, Open Source
It’s Java API to merge XML document created with MS Office (docx) or OpenOffice (odt), LibreOffice (odt) with a Java model to generate report and convert it if you need to another format (PDF, XHTML…).
Snowbound Imaging SDK
Closed Source, Pure Java
Snowbound appears to be a 100% Java solution and costs over $2,500. It contains samples describing how to convert documents in the evaluation download.
OpenOffice API
Open Source, Not Pure Java – Requires Open Office installed
OpenOffice is a native Office suite which supports a Java API. This supports reading Office documents and writing PDF documents. The SDK contains an example in document conversion (examples/java/DocumentHandling/DocumentConverter.java). To write PDFs you need to pass the “writer_pdf_Export” writer rather than the “MS Word 97” one.
Or you can use the wrapper API JODConverter.
JDocToPdf – Dead as of 2016-02-11
Uses Apache POI to read the Word document and iText to write the PDF. Completely free, 100% Java but has some limitations.