Why does converting a PPTX to PDF on Mac sometimes create huge files?
Sometimes you might experience an unexpected, and occasionally dramatic size increase when saving your PowerPoint presentation as a PDF on Mac. In this post, we attempt to demystify this strange occurrence and give you some tools to help when it happens to you.
We first heard about this from a customer who had been troubled by this problem, and thought it might be an issue with our compression software NXPowerLite.
They had compressed their PowerPoint file before exporting it to PDF and had been surprised to find the PDF was much larger. They had to compress it again with NXPowerLite before they could email it.
On closer investigation we came to realise that something a little odd was going on. So we decided to run some experiments and share the results.
Why are some PDF files bigger than PPTX on Mac?
We discovered that the main reason for bloated PDF files created from PowerPoint appears to be due to a little-known limitation of the PDF creator on Mac (“Quartz PDFContext”).
Under certain circumstances some images which are saved as JPEG in the source presentation, are encoded into the PDF using a lossless compression format called Flate when it’s used in PDF files (its official name is Deflate).
Being lossless, Flate retains all of the image information but the downside is that the image size will be a lot bigger. So for example, a single highly compressed JPEG image in your PowerPoint file can easily double in size when it is exported to the PDF and saved using Flate compression.
To test this theory we created a simple presentation containing one slide, with a single 6.2 MB JPEG image, resulting in a 6.3 MB PPTX file, which you can see here:
We then exported this file to create a new PDF which was also 6.3 MB. So far so good.
We then added a design effect to the image (see below) in the original PPTX file and saved it as a new file (design-ideas-presentation.pptx) - this new PowerPoint file was also 6.3 MB.
However, when we saved this version as a PDF it inflated to 10.7 MB! A significant increase. As you can imagine, if you have a lot of these images in your presentation the increase in size can be huge!
This issue seems to be triggered by only certain images and image effects used in the original PowerPoint file, which can trip up the conversion. We’ve seen this most frequently when using PowerPoint’s ‘Design Ideas’ feature (Design > Design Ideas on the ribbon) as shown above - but there are undoubtedly other image treatments that may trigger this issue.
We tried a lot of different designs and although some increased the size, strangely some also ensured the PDF created was smaller. We haven’t established a pattern as yet, so it will be a case of trial and error if you have stumbled on this.
Whilst you can try and edit your presentation to remove images or effects that are triggering the increase in size - that can be very frustrating and time-consuming. So we have a few suggestions on ways to deal with this more easily.
How to deal with large PDFs created by PowerPoint on Mac
Use a PDF Compressor
You can easily compress large PDF files using our PDF compressor for Mac. The 10.7 MB PDF file from our example, reduces to 132 KB when compressed this way.
Create the PDF on Windows
If you have access to a Windows machine, you can create the PDF from PowerPoint on Windows - it doesn’t have the same problem. The example PPTX with the design ideas creates a 198 KB PDF file from PowerPoint on Windows.
Use an online PPTX to PDF converter
Use Adobe’s online PowerPoint to PDF converter. The design-ideas presentation converted to a 334 KB PDF using Adobe’s service.
Could embedded fonts be making your PDF big?
We’ve heard and read about this being a problem but in our experience these don’t inflate the size noticeably when converting from PPTX to PDF on Mac. If you are uncertain however, a surefire way to check this is with our online PDF analysis tool. This will give you a breakdown showing exactly what is taking up space in your PDF, like this example below:
