http://www.lucasfonts.com/fonts/thesans/

TheSans Office (4 Styles)
The four fonts are style-linked and optimized for on-screen reading and word processing in office environments.

Probably TrueType or TrueType flavored OpenType

TheSans (16 Styles)
Cyrillic, Greek, SmallCaps, mutiple figure sets, ligatures, stylisitc alternates, symbols, arrows

Probably Type 1 or PostScript/CFF flavored OpenType

I'm confused ;-) If the pdf document uses The Sans Office and some graphic designer must continue designing the magazine and the font files aren't available, could he use The Sans or should he buy The Sans Office also?

As I understand it it is just a regular The Sans, but slightly adapted (not in appearance!) to make it work in Microsoft Office better? The designer can use just the regular The Sans in Indesign?

To my knowledge the Sans in OpenType has the special figures (0 -9) that are used in TheSans Office. I'm not sure that these figures are also in the PS version. So if you have the OpenType version, you can use that one, but mind the figures!