PDF is my usual choice, but I do sometimes use Word .doc files.
PDF files cannot be easily edited or modified, although you can add comments to them or edit them if you are really determined.
There's no real answer for general, semi-formal document sharing. It falls down to personal preference unless you have some specific requirements in mind.
Both document types are a standard and both are almost universally supported. You could always offer both...
Why not see which one offers you the best bandwidth consumption ?
If your .doc files are smaller than your PDF's, that could help you to make a decision.