Hello @hoc
I believe this is expected result since the ONLYOFFICE native format is OOXML. We recommend converting files to docx/pptx/xlsx formats before the editing process to avoid possible issues: ONLYOFFICE Api Documentation - Opening File The document editing service downloads the document file from the document storage service using the ID and link provided. At this step the conversion of the file into Office Open XML format is also performed for the document editor better performance and formats compatibility.