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just guessing ... Doc/docx are worldwide defacto standard formats, and you want to save as doc because you need interoperability with Microsoft Word, right? However, LibreOffice might be not the perfect tool for that, because its compatibility with Microsoft Office is not good in both directions - screwed up formatting is quite common.
I recommend using SoftMaker FreeOffice instead, that's an office suite made in Germany which is free of charge, too. It has a much better compatibility with Microsoft Office than LibreOffice, means: if you save your documents as doc, they will look faithfully with MS Word and vice versa.
This office suite has several more merits, such as its small size, speed, reliability, or its nice user interface. If LibreOffice fails your expectations, try it out. You can download it here in a Windows or Linux version:
You need to go into TOOLS>OPTIONS>LOAD/SAVE>GENERAL. In option menu there, you should see “Default file format and ODF settings.” Under the dropdown menu for “document type” choose “text document” and in the dropdown menu below that you can choose the default file type to save your documents.