It is possible to upload documentation or a print document for a model. Choosing to first upload a documentation file, you simply do that by clicking Upload Documentation in the Publish tab under the section Print Documents (Web Client). A dialog opens where you have to choose the documentation file you want to upload.
To "upload" a print document you first create and then upload it over the print dialog. Navigate to the Main tab and click the Print button in the Model section.
There you basically configure the model like you would do when printing it and by clicking the Upload Print Document in the left bottom corner the document is uploaded.
Going back to the Publish tab the Download Print Document is now clickable but was grayed out before.
Note: Please note that the print and documentation file is only uploaded per model version. So when uploading a print document while someone is still working on the process the version of the model changes but the new version has no print document attached as the upload took place for an older version. Therefore, it is recommended to finish the process first and then upload the print or the documentation file for the final process version.
The uploaded documents are especially for the Process Portal where you can have a look on every published process and their attached documents as well.