This app enables users to edit office documents from ownCloud/Nextcloud using ONLYOFFICE Document Server. Currently the following document formats can be edited with this app: DOCX, XLSX, PPTX, TXT. The above mentioned formats are also available for viewing together with PDF and CSV. The edited files of the corresponding type can be converted into the Office Open XML formats: ODT, ODS, ODP, DOC, XLS, PPT, PPS, EPUB, RTF, HTML, HTM.
Instruction how to install see here.