If the files you are sending/receiving are Pages files, you will have formatting issues when transferring between Mac and Windows. If you plan to edit these documents, you should save them as Word documents. Word saves in .docx format which is more widely read by Office programs...you can even upload the Word document to Google Drive and edit the document online without worrying about transferring between computers! If you must have a downloaded program, a good free Office program is OpenOffice. It will read Word documents very well.
To save a Pages document as a Word document, open the document in Pages and go to File > Export To... > Word...
If you do not plan on editing the files and are just transferring them for viewing purposes, you should export the Pages document to a PDF document. PDF documents are widely used for viewing documents, can be viewed in a variety of programs, and most always retain the formatting.
To save a Pages document as a PDF, open the document in Pages and go to File > Export To... > PDF...