If you mean you've created a PDF from OOo and you want to know why Word can't open it, it's the same answer. PDF files are meant to be opened by PDF readers such as Adobe Acrobat and the many others.
PDF is not a dynamic file format like a word-processor file, inthat you can edit save, re-open, edit save etc. It is best thought of as an electronic 'print' of the document.
To share files with Word users, use File > Save As and select 'Microsoft Word 97/2000/XP' from the 'Save as type' dropdown menu. Make sure the 'Automatic file name extension' option is checked.
It is always safer to work in the native OOo formats (.odt and .ods etc) and then use Save As to save a copy in .doc or .xls format if you need to distribute it in MS Office formats. But always then go back to the native OOo version to continue working. If the people you are sending to do not need to edit the files then by all means send them a PDF as then no matter what office suite they use it will be possible for them to read and print the files, with a PDF reader.