The answer of the question depends, what you want to do with the data.
The easiest way to export all data from an xowiki instance is to point you browser to xowiki/admin and press export. The result is a plain text file containing all data as xotcl objects. These objects can be loaded into plain Tcl to do with it whatever you want. This is the most flexible approach and allows to import the contents of an xowiki instance into other systems and as well to import data from other systems into xowiki.
If you need a more text oriented approach, you will probably need pages in a certain order. One can use the prototype page "book" and produce a single HTML file from all pages of the xowiki instance, including a table of contents. This requires to specify the page order (section) to define the order and which pages should be exported.
For example, the static HTML file http://media.wu-wien.ac.at/download/xowiki-doc/ was generated this way with minor editing on the result.
One can load the generated HTML file into OpenOffice and save it as ODT. If you want to go towards pdf, there are several options to go from HTML to pdf as well.
