It seems Powerpoint is compressing the images in my slides (each slide in my powerpoint is 1 large image). I have turned off image compression in the advanced settings.
If you're using Powerpoint 2007 or 2010 then the max DPI it will save a PDF at is 220, even if image compression is turned off.
You could install PDF printer software like PDFCreator then "print" your presentation, make sure to check the High Quality checkbox. In PDFCreator click Options then Formats -> Compression and change the compression from Automatic to the level of compression you want e.g. JPEG-Minimum. I wouldn't recommend completely turning off compression as this will create a huge PDF document.
Note that the export bitmap resolution may be increased to a maximum of 300DPI by making the following registry change for the corresponding version of PowerPoint:
Add a new DWORD value, ExportBitmapResolution and set it to decimal 300.
Full info from Microsoft at : http://support.microsoft.com/kb/827745/en-gb