Docx document is read-only in LibreOffice

I created a docx file in LibreOffice on my computer running Debian 9.1 with KDE and copied it to my Android phone. On the phone I try to open the file with the LibreOffice app. However I can't edit the file and just read the document as it's read-only - I get: > This file is read-only, saving is disabled. I already tried a couple of other formats and doing File->Save a Copy on my computer. How can I get it to be editable?