Sometimes it is needed to convert some modern office formats (docx, xlsx) to something that can be read by older devices (odt, ods, rtf, doc, xls,..). The LibreOffice can be used for that. There are some discussions related to the topic.
So if you have a LibreOffice Calc (and an Unix shell) installed then you can convert a bunch of XLSX files to older XLS with this script:
for aa in *.xlsx ; do localc --nologo --convert-to xls $aa ; done
Obviously, the localc is the LibreOffice Calc program. Other programs can be used accordingly (for DOCX the Writer, for PPTX the Impress):
for aa in *.docx ; do lowriter --nologo --convert-to doc $aa ; done for aa in *.pptx ; do loimpress --nologo --convert-to ppt $aa ; done