I think the problem is that you colleague does not have a reader installed which is needed to convert to dropped file.
In the background PDF24 uses the PDF printer to convert the files, but if no reader for that type of file is installed then PDF24 can't print the file on the PDF printer and conversion fails.
Some image files are supported without any other third party software. If you want to convert a Microsoft Word file to PDF then at least the Word Viewer is needed so that the file can be printed on the PDF printer.
If no Word file reader is installed on you system then conversion will fail. PDF24 does not install you readers for any type of file, that is nearly impossible. The PDF printer is the central element here which is used to convert and load a file. Then you only need a reader for that type of file and so you can load nearly every file.
Or just open the file in your reader, click print, choose the PDF24 PDF printer, print the file and that will do the same, you will get a PDF version of your file. Is a little bit more to do, but often works if the auto conversion fails.