You can open a document saved in another format by using the following procedure:
Choose File - Save as.
Select a format from the Files of type list.
Select a file name and click Open.
If you always want the file dialogs to show another format by default, choose LibreOffice - PreferencesTools - Options - Load/Save - General and select that format as Default file format.
Converting all documents of a folder
Open the wizard, which guides you through the operation, to copy and convert all documents from Microsoft Word, Microsoft Excel or Microsoft PowerPoint into OpenDocument file format documents. You can select a source and target directory, specify whether to convert documents and/or templates, and more besides.
Opening HTML files in Writer
Choose the file type "HTML Document" to open in LibreOffice Writer/Web. This is the default for HTML documents in LibreOffice.
All the options of LibreOffice Writer/Web are now available to you, such as Show HTML source.
Choose "HTML Document (LibreOffice Writer)" to open in LibreOffice Writer.
All the options of LibreOffice Writer are now available to you. Not all options that LibreOffice Writer offers for editing of documents can be saved in HTML format.