Some time ago, I discovered that there might be a better way to deliver online presentations with voice. Having the audio and the right advance times added to the slides would generate way smaller output files without any quality loss.

In some sense, these presentations could be handled as extremely simple animations. This is why Flash is a suitable format for them—bear with me, let’s put Flash deprecation aside for a moment. Also PDF includes both multimedia esedding and scripting, that allow to advance the slide when main audio has reached a given point.1

Mandatory Sample

Just in case you wonder what I’m speaking about, here you have an awesome introduction to ancient Greek accentuation rules2:

There PDF document is available here to download. Only the latest version of Acrobat in Windows is able to deal with automatic slide advancement.3

Some Numbers

Comparing file sizes from the original video and resulting output formats, gives the follwoing results:

Format Total Image Audio
MP4 30 MB 15.1 MB 14.7 MB
PDF 2.4 MB 476 KB 2 MB
SWF 2 MB 30 KB 2 MB

Depending on the format, the resulting numbers are way smaller4.

Converting Slides

Being mainly a text presentation, conversion from PDF to SWF is pretty efficient. File size is decreased to a fifteenth of the original one—although this may be an extreme case5.

The final Flash presentation is also a fifteenth in size of the original video. But of course, this wouldn’t be impossible without some audio conversion.

PDF document isn’t converted when the final format is also another PDF document. Unless the document that contains the original slides is poorly generated, the file size for the slides in the final document won’t change.

Flash Deprecation?

Both the Flash file format6 and the Flash player were developed by Adobe. They announced that the player7 will be deprecated after 2020. But the format is still valid and there are some projects that allow the playing of Flash files.

Ruffle is an open source project that provides desktop programs for Windows, macOS and Linux, and also extensions for Firefox and Chrome. They even provide an extension to be hosted on sites that contain Flash files—in that case, only a fairly decent browser is required.

Ruffle is a project in development. Since the kind of presentations I’m speaking about are pretty basic, the current status of Ruffle allows to play these presentations—and even to use loaders.8

Tweaking Audio for Voice

There is also a huge size decrease in sound, because recorded human voice doesn’t need so much data as music.

Original sound is recorded at 256 kbps. For compressed audio, this results in good music quality. But I doubt whether this makes sense for recorded voice.

MP3 is the only compressed audio format that both Acrobat and Flash allow. 32 kbps is a reasonable quality for recorded voice. This makes the main decrease in sound file size.

For PDF only, Opus could be used with a quality of 16 kbps. This would lead to a sound file half in size—1 MB, in this case. But Acrobat cannot deal with audio files in Opus format.


I think the main benefit of this approach is the much decreased file size in the final output.

This main benefit leads to other ones:

  1. No buffering, since the whole file is probably the amount of data needed for video buffering.

  2. No decrease in image quality when you have slow connectivity.


  1. This is the main mechanism for both formats: check sound position and advance slide (or frame) when the position is equal to the one defined for eah slide. 

  2. The original video is located at

    James Tauber is the author of that video, which has been released under a Creative Commons Attribution–ShareAlike license

  3. On macOS, QuickTime doesn’t seem to report position from the media it plays. Acrobat plays media using QuickTime, so it will play sound from the presentation, but it is impossible that it will advance the slides. 

  4. Anoter presentation would be First Do No Harm by Mary Ann Davidson (

    Format Total Image Audio
    MP4 151 MB 108 MB 40.9 MB
    PDF 10.7 MB 666 KB 10 MB
    SWF 10.9 MB 1.1 MB 10 MB

    This is a sample only with numbers, since video, audio and slides are copyrighted. 

  5. There may be some file size decrease in the PDF to SWF conversion, but it is around a half in a more standard lucky case. 

  6. Technically speaking, SWF stands for ShockWave Flash and it is the extension name for Flash files—.swf. The SWF File Format Specification can be found at

  7. After 2020, it seems that the player will be only supported in China and on some entreprise platforms worldwide. 

  8. To avoid loading files over 1 MB with the main page where they are located, loaders are about 10 KB in size. Once clicked on them, the main presentation will be loaded.