Very well possible given that likes of POI are written using Java(or can be). Basically, xlsx files are zip files. If you change the extension to .zip file. You can view some bunch of xml files. Look at Office Open XML. Office Open XML (also informally known as OOXML or OpenXML) is a zipped, XML-based file format developed by Microsoft for representing spreadsheets, charts, presentations and word processing documents. The format was initially standardised by Ecma (as ECMA-376) and, in later versions, by ISO and IEC (as ISO/IEC 29500). You may follow that standard to write your own library in java to extract the data from excel files and follow pdf standard to convert the same into pdf format without using any external libraries. If you mean if there are any inbuilt packages in JDK, no. But if you mean if it can be done using Java, yes. Because pretty much anything can be written from scratch. But, it may not be a good idea to reinvent the wheel.
If nothing else, Excel is well documented, and it's a pretty easy format to work with.