Making sure your Word, PowerPoint, and PDF files do not become excessively large is important to ensure that your materials are easily accessable to students. Students sharing a busy wireless connection in the lecture hall or using a dial-up connection at home will appreciate the effort you spend to optimize your course materials.
If you have Adobe Acrobat Professional installed, use Acrobat's built in Optimizer tool to effectively reduce the size of your PDF.
In the PDF optimizer, pay special attention to the Image Settings. To select a proper pixels/inch resolution, consider what types of images you have. If your images consist of photographs or diagrams with large labels, use 72 pixels/inch; if there is small text in your diagrams, use 144 pixels/inch. In both cases, "Medium" quality will produce acceptable results. Review your document before releaseing it to students, to make sure all diagrams are still easily readable.
If the optimized PDF is unreadable because of noise (seemingly random stray marks) around text, lines, or edges in diagrams, try changing the quality to "High". If the image just appears blocky, try increasing the pixels per inch. A higher number will preserve more detail in the PDF, but it will also make the image larger.
If you use another application, such as Apple's built-in PDF maker, to convert a Word or Powerpoint file into a PDF, review the instructions for those file types before saving as a PDF, as this will affect the size of the PDF produced.
Be aware that PDFs created by scanning documents are not automatically accessible to low-vision/blind users. However, you can take steps to make your PDFs accessible to all users.
Microsoft has posted a good tutorial describing how to decrease the size of Powerpoint files.
While most of the article applies to Mac OS as well as Windows, the Mac OS version of Powerpoint does not allow you to compress all images at once. To compress an image, when you get to the Format Picture dialog box, click the Size tab, and put a check next to Best scale for slide show.
An excellent guide to reducing file sizes in Word files.