What is a PPTX file?

PPTX is a container: a wrapper that holds streams encoded by something else. It is used for editing.

The extension is .pptx, and the full name is PowerPoint Presentation. Both matter less than what the file can hold, which is what the rest of this page is about.

Microsoft published it in 2007. The specification is ECMA-376.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

PPTX is not limited to one page, which matters when converting to something that is: one conversion produces one file holding a single page — the first one, unless the page is something the converter lets you pick.

PPTX cannot carry executable code, which is why mail gateways and document portals let it through where they stop its macro-enabled twin.

PPTX holds a presentation: one slide per screen, sized for a projector rather than for paper.

That is what decides how it converts: one page per slide, with the animations flattened, because a page cannot appear in stages.

Microsoft PowerPoint, LibreOffice Impress and Keynote read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

PPTX is meant to be opened and changed. Keep the file in this format for as long as the work is going on, and export from it whenever a finished copy is needed.

PPTX is an Office Open XML package: a ZIP archive containing an XML document for each slide, more for the masters and layouts, a relationships file recording what refers to what, and a media folder holding every image, video and sound.

That structure explains most of the format’s behaviour. It is why a deck of forty slides opens quickly — only what is needed is read. It is why file size tracks the media folder rather than the number of slides. And it is why a corrupt deck is often recoverable: the pictures are sitting in a folder inside the archive at full resolution.

A deck has slide masters, each with several layouts, and every slide is an instance of a layout. Change the master and every slide follows. That is the mechanism, and it is the one people fight rather than use.

The symptom is familiar: a heading that returns to the wrong font, a logo that appears on slides you did not put it on, a colour that keeps reverting. All of that is the master asserting itself over local formatting. Fixing it slide by slide is the slow way; changing the master is the fast way, and it is also what stops the problem coming back.

The related mess is pasting slides between decks. A pasted slide brings its own layout with it, so a merged deck accumulates masters, and the theme picker fills with near-identical entries. Pasting with "use destination theme" avoids it.

Images, essentially always. A photograph dropped in from a phone is several thousand pixels wide and is stored at full resolution even though it is displayed at the size of a postcard, and twenty of those is a deck nobody can email.

PowerPoint has a compress-pictures function that resizes to the resolution actually needed and discards cropped-away areas — which is the second surprise, because cropping in PowerPoint hides the rest of the picture rather than removing it. A deck can therefore contain photographs nobody can see, at full size, and in a confidential presentation that is a disclosure problem as well as a size one.

Video and audio can be embedded in the file or linked to a file on disk. Linked media is not in the deck, so a presentation that plays perfectly on your machine has silent gaps on any other — the classic failure ten minutes before a talk.

Even embedded media depends on the codec being available where the deck is opened. The reliable arrangement for a talk you are not presenting yourself is to send a PDF and supply any video separately, or to present from your own laptop. No presentation format guarantees a video will play on a machine you have never seen.

A slide has no reflow to absorb a substituted font. Open a deck on a machine without the typeface and every text box is laid out with something of different widths — headings wrap onto a second line, captions overflow their boxes, and carefully aligned elements stop aligning.

PowerPoint can embed fonts, which fixes it and grows the file and depends on the font’s licence allowing it. The alternative is to stick to typefaces that exist everywhere, or to accept that a deck handed on is a deck that may look different. PDF removes the question entirely, which is why it is the right thing to send for anything final.

PPT is the binary format PowerPoint wrote until 2007. It is worth converting anything still in it: PPTX is smaller because it is compressed, it is a documented standard rather than a reverse-engineered binary, and it cannot carry the old macro viruses that made PPT files a security concern.

ODP is the OpenDocument equivalent, native to LibreOffice Impress. Conversion in both directions handles text, layout, images, tables and charts well; animations and transitions map approximately or disappear, and embedded media is the least reliable part. If the recipient uses Impress and needs to edit, send ODP — otherwise PPTX is what the world has.

Rename to .zip and open it: every image is in a media folder at full resolution, which is far better than screenshotting a slide, and the slide text is in readable XML. This also works when PowerPoint refuses to open a damaged file, and it is the recovery route worth trying before anything else.

For sharing, the export options matter more than people use them. Notes pages produce a document with each slide above its speaker notes, which is the right personal reference for a talk. Handouts put several slides on a page for circulating afterwards. And exporting to PDF is the one that guarantees the recipient sees what you saw.