LibreOffice

LibreOffice is a very feature-rich free MS Office alternative suite without an Internet connection.  It is the default office suite in the majority of Linux and is developed under GNU’s LGPLv3 and the Mozilla Public License (MPLv2). It suitable for government and other security-conscious offices. opens almost any legacy document.

I've been using LibreOffice ever since it forked from OpenOffice. It's one of the best office suites out there. For the most part, LibreOffice and Microsoft Office have the same suite of software except Outlook. So there is no email client built in. If you need Outlook or an equivalent, like Evolution which combines e-mail, a calendar, and an address book.

Windows 10 PCs come with a trial of Microsoft Office installed, but this will expire, leaving you with no office suite. However, LibreOffice for Windows is also free for creating text documents, spreadsheets, and presentations which you can then save in Microsoft Office formats. With LibreOffice, you get nearly all everything of Microsoft Office without the hefty price tag.