There are several worksheet file types you may come across on BusyTeacher.org.
1) PDF: one of the most common document formats on the Internet. Chances are, you already have a PDF viewer installed on your system. If not, you can download and install free Adobe Acrobat Reader, or a smaller and faster alternative: FoxIt PDF Reader.
2) DOC: this is a usual Microsoft Word document, please use Microsoft Word to open it.
3) PPT: this is a Microsoft PowerPoint document, it can be opened with Microsoft PowerPoint.
4) DOCX: this is the Microsoft Word format used by the later MS Word versions (MS Word 2007 and MS Word 2010). If you ca't open such file with your MS Word, there are a few ways out: download and install a Microsoft Compatibility Pack which adds support for such files, or use an online DOCX2DOC converter.
5) RAR/ZIP: this is a standard format for archives across the Internet. If your system does not have a default way to unpack such archives already, please google one of the following tools: WinZIP or WinRAR.