PDF is the document format of choice for a number of reasons, but not everyone with whom you decide to share a PDF will be able to modify it (which is kind of the point). For those who need to, CometDocs is a free, online file converter with the ability to convert between over 50 different file types, including PDF, ODF, DOCX, HTML, RTF, XPS, and a variety of image formats.
File conversion with CometDocs is pretty simple, and if you want high-quality image-to-text OCR support, there is a paid, premium version (although the free version does support some primitive OCR). One nifty feature is the ability to discern the file type of a file with a misplaced extension.
CometDocs can even compare two different documents of differing types line-by-line and return a print-out of where they differ. One caveat: if there is any sensitive information in the documents you wish to have converted, stick with a good desktop-based file converter. Online file converters typically do not encrypt the transferred data, and there is nothing on the CometDocs site that states otherwise.
If you need a secure solution, but you want something online, Zamzar offers 128-bit encryption uploads and supports a good number of file types (video and audio included) for $49 per month. Unfortunately, ZamZar is not as feature-rich as CometDocs.