It is strongly advised to save in the .odt format, always. It’s the native format of LibreOffice, and .doc and .docx aren’t. Which means that Libre will always convert a .doc(x) file to its native format on opening, modify it in that format according to user input, and convert back to .doc(X) when it’s saved. So, you waste time and system resources by converting.
The exception to the rule is when you have to send a file to somebody who insists on .doc or .docx. In that case, edit the document in the .odt format, then when it’s finished, save in the .odt format and only then save in .doc(x) format. If something goes wrong with the .doc, you still have the original .odt.
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