You can now import from PowerPoint and PDF. More info and instructions available here: http://help.slides.com/knowledgebase/articles/487268-import-from-pdf-or-powerpoint
So far this is limited to image-based importing, meaning the inner content of imported slides can’t be edited. We’re hoping to add editable imports at a later point.indolering commented
This task is very difficult: proprietary software tends to produce a lot of nasty markup (often times on purpose to make interoperability more difficult) and there are often basic impedance mismatches between the platforms. Unless software exports cleanly, it is very hard for anyone to support importing their content! 90% of the time it is better to just recreate documents.