In Word and PowerPoint for Mac 2011, you can change the compression behaviour for a placed bitmap image via the `Format Picture` ribbon. Double-click the image to bring that ribbon to the front, then click the `Compress` button and choose the option `Keep Current Resolution`. Or, choose `File` > `Reduce Size` to bring up the dialog box. This needs to be done **before** the presentation is saved with the newly placed image.

It's different for vector graphics from PDF files, which PowerPoint will simply convert into bitmap graphics and compress as soon as the presentation is saved with the newly placed PDF file. This can be avoided in MS Office by using vector graphics in the **.EPS** file format or in Microsoft's own **.EMF** format, which can be exported from most vector graphics editors, e.g., Inkscape, OpenOffice Draw, Adobe Illustrator, etc. (Beware: PDFs exported from bitmap editors like GIMP, Photoshop, or Pixelmator are not vector graphics images.)