Super high resolution images of small slide elements can inflate the size of your PowerPoint or Keynote file without you noticing. A common culprit is an innocent looking page with 30 customer logos. Compress your images often to keep file sizes in check.

Another common file size mistake is to include high resolution images in the slide master to make it easer for people to understand template slides that are meant for photos. As a result, even a simple text slide will create a huge file as the slide master gets saved as an integral part of the document. This can add up in a company with 10,000 employees.

Image compression in PowerPoint can sometimes produce unpredictable results, especially when you tick "apply to all" and you have a presentation with a lot of photographs. I often see cropped images going haywire, the only rescue is to compress images one by one. Always save a copy of your file before attempting to compress the file.

Handy link: how to reduce file sizes in Office

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