Make a PowerPoint presentation (.pptx) smaller: every picture is reduced to the sharpness its place on the slide needs, and photos stored as PNG become JPEG. Text, shapes, notes, animations and everything else stay exactly as they are.
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Only the pictures. Every picture is reduced to the sharpness its largest place on the slides needs (220 pixels per inch for printing, 150 for screen and projector or 96 for e-mail), like 'Compress Pictures' in PowerPoint. Photos become JPEG; a logo, a drawing and a picture with transparent parts stay lossless. A picture that does not get at least 10% smaller stays as it was. Text, shapes, notes, animations, charts and videos stay in the file exactly as they were.
A picture that also appears in a chart, SmartArt, notes, a table or a placeholder, or that is repeated as a pattern: there the tool does not know how large it is shown. Also EMF, SVG and GIF, CMYK photos, and pictures that are no sharper than their place needs. After you choose your file, the tool says how many can get smaller and why the rest stays. A portrait phone photo stays upright: the rotation is kept. The other data of a photo (camera, location) is not kept.
No. Everything happens in your browser: your presentation never leaves your computer. Your own file does not change either; you get a new presentation with '-smaller' in its name.