You can view documents, spreadsheets, and pdf files on your Kindle Fire. To transfer them onto you kindle, you can email them to your Kindle email address. Your Kindle email address can be found by going to Settings then My Account on your Kindle Fire or on the Amazon web site. Alternately, you can transfer files onto your Kindle Fire by attaching it to your computer via the USB cable and copy the files to the Documents folder on your device. Once your files are on your Kindle, you can view them by going to the Docs section or you can use an application like Office Suite to view them.
This feature can also be used to receive documents and files from any email address on your approved email list. To access and manage this list, go to the Manage Your Content and Devices page at Amazon then go to Setting. I use this method to receive free ebooks formatted for Kindle from publishers like Baen who maintains an excellent free library.
Advanced Tip: When Personal Document Archiving is enabled, personal documents sent to your Kindle email address will also be added to your Kindle library.