Open a .pptx or .odp presentation
A presentation arrives by email and double-clicking does nothing: on a phone, on a Mac without Office or on a computer with no licence you need an app that wants you to sign up. Here you read the slides straight away, with the speaker notes, and take them with you as text. The file stays on your device.
Why a .pptx does not open on its own
Because it is not a document, it is an archive: inside there is one file per slide, plus the images and the links between the pieces. The browser has no built-in reader and the programs that do open it often want an account, so the route everybody takes is uploading a work presentation to some website. Here the archive is opened in your browser: the slides are put back together one after another, with their titles, lists, tables and pictures.
The speaker notes, which almost nobody shows you
Under every slide there may be what the presenter was meant to say: those are the speaker notes, and they live in a separate file that most viewers do not even open. Here you see them, and that helps twice over: if you have to study the presentation, the notes are often the densest part; if you are the one sending it on, it is better to know what is written down there before somebody else reads it.
The order of the slides is not the order of the files
Inside the archive the files are called slide1.xml, slide2.xml and so on, but that is not the order of the presentation: the real order is written separately, and the moment somebody moves a slide in PowerPoint the two orders part company for good. A reader that goes by file name shows the slides in the wrong order without anybody noticing, and it is a mistake you cannot spot by eye: here the real order is the one that is read.
What you see and what you do not
This is not a viewer: there is no layout, no original fonts, colours, animations or templates. There is the content, which is what you need nearly every time: titles, text with its levels, tables, pictures and notes. Where there is a chart it says so, instead of quietly making half a slide disappear: the numbers are in the file, but the drawing is done by PowerPoint. The text is real text, so you can select it, copy it and search it.
OpenDocument too, and what it does not do
It also reads the .odp, that is the presentations of LibreOffice and OpenOffice, which have the same problem the other way round. It does not read the old .ppt of PowerPoint 97-2003, which is a completely different format: in that case it tells you, and whoever sent it can save it as .pptx in ten seconds. It changes nothing and does not convert to PDF. If the file you have is a .docx, a .zip or an .xlsx, there are pages made for those.