The auto conversion of documents simply prints some documents on the PDF printer to convert them to PDF.
PDF24 can't convert all documents out of the box, that would be fantastic, but there are a lot of documents, that would be very hard to realize.
The trick is to use the PDF printer to convert the documents. Normally the documents can be printed on a printer and if so the document can be converted to PDF. This way PDF24 does not need a reader for every document type. The reader can be installed by the user and if the reader can print a document then a PDF can be generated.
Out of the box PDF24 supports some image type, but that's all. In all other cases the PDF printer must be used and this can be greatly automated internally.
That's the reason why the printer is used.