If you don't need Microsoft Word like "Sinhala" font support, I highly recommend "Free version" of WPS Office.
It's interface is very similar to Word and features also very similar to it, it supports all word file formats.
Only issue I found is it's sinhala font support is not as good as Word.
If you create/edit sinhala documents, use msguide method suggested by another member.
else use WPS Office.