Until eLML 3 only one lesson (or technically spoken: one XML file) could be transformed at the same time. Since eLML 4 it is possible to transform multiple lessons into one course no matter what the output format is. So technically spoken you can transform multiple XML files into one PDF file or one content package (ZIPZIP file). All you have to do is define a so called "course" in your configuration file. Here is how it works:
Before transforming a lesson eLML checks if the label name of the actual lesson is part of a course defined in the projects config.xml file. If not, the XML file is transformed alone. If it finds a course that contains the lesson label, eLML starts by transforming the first lesson listed in the course and will continue until it reaches the last lesson label. This works for every available format and of course eLML will also create a continuous numeration if you activated the numeration of chapters. The following example shows you how a course is defined in your config.xml. This course contains the four GITTAGITTA database lessons IntroToDBS, DBSysConcept, LogicModelin and RelQueryLang (in this order). To see how this course looks in the PDF version click on the PDF navigation screenshot on your right and download the database course as a PDF file.