I need to combine those documents. Can you do it with LibreOffice? I could not find anything in Edit > Compare documents because such a thing does not exist anymore.
I could not find a way to do from *.docx to file.docx, but you can do to .pdf
soffice --headless --convert-to pdf *.pptx
System: Ubuntu 16.04
Open the first docx document and go to the end of the document, select Insert\Document… then select the docx document you want to add. This will add the new document to the bottom of the original one. You can add the document anywhere in the original one, just click the place so the cursor is flashing where you want to add it.