How to Compress a PowerPoint on Mac

Pick by what is inside the deck, not by which app you own. If there is no video, the first row solves it and you can stop reading.

Two of those rows are marked “not measured” and stay that way. Microsoft Office is not installed on the test machine and Keynote’s reduction sheet has no scripting interface, so quoting numbers for either would mean inventing them. What both commands do is documented by their vendors and linked below.

Because a slide does not store the picture you see. It stores the picture you dropped in, at whatever resolution it arrived, and then draws it small. A 4,032 × 3,024 phone photo shown six inches wide on a slide is still 4,032 × 3,024 in the file.

You can verify this on your own deck in one command. A .pptx is a ZIP archive of XML and media, so unzip -l lists everything inside it without extracting anything:

unzip -l deck.pptx | sort -rn | head -12

On the test deck that printed:

One detail worth knowing because it saves you chasing the wrong thing: the test deck used photo1.jpg on two different slides, and the archive contains exactly one copy of it. Reusing an image across slides does not duplicate the bytes. Pasting the same photo twice from your Photos library might, because they arrive as separate files.

There is also a ppt/printerSettings/printerSettings1.bin in there. It is 9,395 bytes. It is not your problem.

It resamples images down to a target resolution and, optionally, throws away the parts of pictures you cropped off but PowerPoint is still storing. Microsoft documents the Mac route as File → Compress Pictures, with a Picture Quality list running from highest resolution at the top to lowest at the bottom, and a choice between Selected pictures only and All pictures in this file.

Two things on that page decide how much you get back. Microsoft states that “the default picture resolution in Microsoft 365 is 220 ppi”, so a picture placed six inches wide is being kept at roughly 1,320 pixels across once compressed. And there is a Delete cropped areas of pictures checkbox, which matters more than the resolution setting if anyone has been cropping photos on slides rather than before importing them. Cropping in PowerPoint hides pixels. It does not remove them.

What Compress Pictures will not do is touch media1.mp4. On the test deck that is 16,114,403 bytes of the 41,010,934 total, and it is the reason so many people run the command, watch the file drop from 90 MB to 60 MB, and conclude that compression does not work on their deck.

Shrink the media and put it back. Because the file is a ZIP, the images inside it can be replaced with smaller versions of themselves and the deck still opens. This is not a trick; it is what every “presentation optimizer” does under the hood.

First the images. The six 4,032 × 3,024 photos went through Smol at 1,600 px on the long edge, quality 75:

At 1,600 px a picture shown six inches wide still resolves 266 PPI, which is past the point anyone will notice on a projector or a Retina screen. That is a 96.7% reduction in the part of the file that was the problem.

Swapping those back into the archive gave three results worth separating:

The middle row is the honest ceiling for image compression on a deck that contains video. You do 96.7% better on the photos and the file still will not go below the size of the movie. The clip compressed from 16,114,403 to 8,852,033 bytes on its own, which is where the rest of the saving in the last row came from.

Be clear about one limit before you reach for a compression app: Smol does not open a .pptx and shrink it in place. Handed the test deck directly, it returns “This file couldn’t be processed. It may be corrupted or in an unsupported format.” It compresses the images and the video that go into a deck, and it converts presentations between pdf, pptx and odp. Those are different jobs and the distinction is worth having straight before you buy anything.

If nobody needs to edit it, yes, and it is usually the single biggest win for the least effort. Converting the test deck to PDF took 4,933 ms and produced a nine-page file of 20,077,857 bytes, down 51.0%. The photos came out at 1,800 × 1,350 at 300 PPI, which is print quality.

Then the interesting part. Running that PDF through PDF compression barely moved it: the small preset took it to 16,353,665 bytes, only 18.5% off, even though it had shrunk every picture to 900 × 675. Reading the PDF’s own structure explains why in one line:

strings deck.pdf | grep -c EmbeddedFile

The exported PDF carries the movie as an embedded file with MIME type video/mp4, in a single 16,114,404-byte stream. After compression a 16,109,198-byte stream was still there. 98.5% of the compressed PDF was the video. No PDF compressor on any platform is going to fix that, because it is not a PDF problem.

So the order matters: deal with the video before you export, not after. If the video is only there because someone wanted motion on slide nine, take it out and link to it. If it has to stay, compress it first — the same reasoning as compressing video on a Mac. Once the deck is a PDF, the routes in how to compress a PDF on Mac apply, none of them need Acrobat, and the field is ranked in the best PDF compressor for Mac.

Keynote opens .pptx files and exports back to them, and it has a reduction command that is more thorough than PowerPoint’s. Apple documents File → Reduce File Size in the Keynote 15.3 user guide, with options to Scale down and convert images, Remove trimmed parts of movies and audio, and a Movie Quality control offering Most compatible (H.264), High efficiency (HEVC) and Preserve original format, plus a maximum resolution. It can apply all of that to the open file or to a copy, through Reduce This File and Reduce a Copy.

That movie handling is the part PowerPoint’s Compress Pictures does not have, and on a deck like the test one it is the whole ball game. We have not published a number for it because the sheet is not scriptable and a hand-run figure on one file is not a measurement anyone should trust. Use Reduce a Copy and compare the two files in Finder yourself.

The catch is fidelity. A PowerPoint deck round-tripped through Keynote comes back with Keynote’s interpretation of your fonts, masters and animations. For a deck you made and will present, that is fine. For a client template with brand typography, check every slide before you send it.

Compress the pictures before they touch the slide. It sounds obvious and almost nobody does it, because the friction is in the wrong place: the photos arrive in a folder and the deck is due.

The whole fix is one pass over the folder. In Smol that is a drag, a size cap, and a quality number; the six-photo set took 24,870,623 bytes down to 823,249 at 1,600 px. Anything that produces the same result works as well — the point is that a deck built from 1,600 px images cannot become a 40 MB deck no matter how many slides you add.

Two numbers to aim at. For screen and projector, 1,600 px on the long edge is generous. For a deck that will be printed at A4 or Letter, 2,400 px is the point past which you are paying megabytes for detail the printer cannot resolve.

A Word file has exactly the same anatomy and exactly the same fix, minus the video problem, which is the subject of compressing a Word document on Mac.

If you have Microsoft Office, start with File → Compress Pictures. It is free, it is in the app the file is already open in, it understands cropped areas, and for a photo-only deck it is very likely the end of the story. Buying anything before you have tried it is a waste.

If you have Keynote and the deck contains video, Reduce a Copy is the better tool than anything we make, because it rewrites the movies as well as the images and keeps the file a presentation. Smol cannot do that; it will not even open a .pptx for compression.

If the deck is final and read-only, export to PDF and be done. That is free from every app that can open the file.

Where Smol fits is upstream and in bulk: the folder of photographs before they become slides, the video before it gets embedded, and the PDF afterwards if the export is still too big for the portal you are uploading to. If that is a weekly job rather than a one-off, Smol is $29 once, runs entirely on your Mac, and never uploads the deck anywhere. If it is a one-off, use the free tools above.