I looked up online on how to set a Powerpoint (for Mac) presentation (2007) to open as a slideshow automatically upon opening. They all said to change or save-as file extension from 'pptx' to 'ppsx', and this will auto open as a slideshow. However, after doing this and reopening it actually opens in Keynote. Even the file is under the 'Keynote' heading in Finder. What is going on? How do I set Powerpoint to open automatically on slideshow when i email to others? Thanks.

Your mac wants to use Keynote to open files with the ppt or pptx extension.
Tell your mac to open PPT, PPSX or PPTX files with PowerPoint instead. In your FINDER find the file you'd like to open with PowerPoint, and select it. Click on the File menu and select Get Info. Expand Open with: by clicking on the triangle to the left. Select the Open with: menu, then choose the app you'd like to use to open all documents like that one.