What is a PPTX file?

One file that is really a folder

Make a copy of any .pptx, change the ending to .zip, and it opens like an ordinary archive. Inside is a tidy tree of parts. A presentation part lists the slides in order, each slide is its own XML file describing the shapes and text on it, and separate folders hold the slide layouts, the slide masters, the theme with its colours and fonts, and a media folder with every picture. Charts carry their data in a small embedded workbook, which is why a chart in a PPTX can be opened and edited rather than just looked at. Speaker notes have their own parts too. Nothing about this is hidden; it is simply packaged so that it travels as one file.

A published standard, not just a product format

The structure is defined in ECMA-376, the Office Open XML standard, first published in December 2006 and now in its fifth edition, and it corresponds to the international standard ISO/IEC 29500. That is why other applications can read and write PPTX with reasonable confidence rather than reverse engineering it. Microsoft lists the practical benefits of the design: ZIP compression that can make files up to 75 percent smaller in some cases, and a modular structure that keeps different kinds of data apart, which helps when a damaged file needs recovering. The OpenDocument format behind .odp files is a separate standard maintained through OASIS, so the two are cousins rather than versions of each other.

Why the same file looks different on two machines

A PPTX describes a slide; it does not freeze a picture of it. The opening application reads the description and draws the slide itself, using whatever fonts are installed. When the designer's typeface is missing, a substitute takes its place, text runs wider or narrower, and a heading that fitted on one line wraps onto two. Charts are redrawn by the opening app, so colours and label positions can differ slightly between PowerPoint, Keynote and a web viewer. This matters most when a deck leaves your hands: a client presenting from a conference laptop, or an agency handing work to an in house team, will see the file their software draws, not the one on your screen. The practical defence is to open the file once on a machine that has never seen your fonts, ideally the one that will run it, and fix any heading that wraps before the day rather than during the talk.

Common mistakes

The first mistake is renaming a file instead of converting it: changing .key to .pptx produces a file nothing can open, because the contents are still Keynote's. The second is saving down to the old .ppt format for compatibility, which almost no current software needs and which can quietly drop newer effects. The third is sending a deck built on custom fonts to someone presenting on an unknown machine without checking how it looks there. The fourth is sending a .ppsx show file to a colleague who needs to edit, who then assumes the deck is locked. The last is assuming every PPTX is editable, when some are exports that contain one picture per slide.

The two PPTX files the slide builder exports

The slide builder exports three formats: PDF, PowerPoint and Editable PPTX, each rendered on the server from the saved version. The PowerPoint export is a picture per slide on the 13.333 by 7.5 inch widescreen layout, so it looks exactly as designed on any machine and nothing on it can be edited. The editable export rebuilds text, lists, images, tables and charts as native PowerPoint objects, with nine of the ten chart kinds becoming real charts and the waffle chart staying a picture. Both PowerPoint exports are blocked on the Free plan, and neither carries speaker notes. There is no .ppt, .key or .odp export. The trade offs between the two PPTX files have their own page.

Questions people ask