The deck bounced off the mail server, or it will not upload anywhere, and nobody can tell you why a talk with twelve slides on it weighs 90 MB. Almost always it is four photographs and a video, and you can have most of the space back in two minutes. To read a deck rather than fix one, the PowerPoint viewer takes PPT, PPTX, ODP and the slideshow, macro and template variants up to 20 MB, and deletes the file when you close it.
Pictures are stored at the size they arrived
This is the big one, and it surprises people because the slide looks fine.
Drag a photo off a modern phone onto a slide and you have added twelve megapixels to the file. Pull the corner handles until it sits neatly in a quarter of the slide and you have changed how it is displayed. The twelve megapixels are still in the file. The same is true of cropping: cropping hides the parts you cut, it does not remove them, so a slide showing one face may be carrying the whole conference room.
The fix is one dialog. Click any picture, then Picture Format, Compress Pictures. Untick "Apply only to this picture", tick Delete cropped areas of pictures, and choose a resolution — 150 ppi is more than a projector or a screen can show. Save under a new name, because this throws the extra pixels away for good.
A video you inserted is inside the file
Insert, Video, This Device copies the entire video into the deck. A two-minute clip can be larger than everything else put together, and it goes wherever the file goes.
If the audience will be online, link the video instead of embedding it. If it must play offline, send the deck and the video as two files and accept that you are sending two files.
Embedded fonts, and the setting that doubles them
Under File, Options, Save there is "Embed fonts in the file", with two choices. "Embed only the characters used in the presentation" adds a little. "Embed all characters" adds an entire typeface per font, for every weight you used, so a deck with a corporate font family can carry several megabytes of alphabet it never shows.
Embedding is worth it when the deck will be opened on a machine that does not have the font. It is worth the smaller option either way.
The leftovers nobody deletes
A deck that grew out of a company template usually drags along slide masters and layouts nothing uses. View, Slide Master, then delete the layouts with no slides attached to them. It is rarely the difference between 90 MB and 9, but it is free.
While you are here: .odp is not .ods
If somebody sent you an OpenDocument file, look at the last letter. .odp is a
presentation and opens in the PowerPoint viewer.
.ods is a spreadsheet and belongs in the Excel viewer.
One letter apart, two unrelated formats, and it is the reason a perfectly good file
sometimes appears to be broken — it was simply handed to the wrong application.
When it still will not open
Over 20 MB after all of that means the deck genuinely is that big: send a PDF instead,
or split it. A file that refuses everywhere may be a .ppsx, which is the same deck
saved to start playing immediately rather than open for editing — the viewer takes
it. The older .pps it replaced is not on the list: open that one in PowerPoint and
save it as .pptx first.
If decks keep landing in your inbox from people who assume you have PowerPoint, the PowerPoint viewer is the page to keep.