Do you only want to read Office documents or also edit them?
If so these three options below are the best options out there outside of the Office apps (
Word,
Excel,
Powerpoint) by Microsoft for which you need Office365 subscription in order to make edits to documents (first 30 days are free). Reading them with the Microsoft Office apps shouldn't be a problem with the apps without a subscription.
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WPS Office - free to open
and edit, with ads, for a version without ads and more features there is a
subscription option.
TIP!
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Polaris Office - free to open
and edit, with ads in free version, for no ads and more advanced features there is a
subscription option.
TIP!
Not as good as the above two but also an option:
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OfficeSuite - limited features in free version, you can edit but you can't save documents in Office filetypes 'xls,'ppt' or 'doc' , with ads in free version, saving documents in 'xls,'ppt' or 'doc' needs a subscription as well