Open the PowerPoint file
Go to File
Select Info
Click Compress Media if available
Choose the desired compression quality
Wait for the compression to finish
Select a picture in the presentation
Go to Picture Format
Click Compress Pictures
Uncheck Apply only to this picture if needed
Choose a resolution option
Click OK
Save the file as a new copy
Remove unused slides
Delete unused speaker notes
Delete unused embedded media
Replace large images with smaller versions
Use linked media instead of embedded media when possible
Save the presentation in PPTX format
Use File
Select Save As
Choose a smaller file location if needed
Close and reopen the file to confirm the reduced size