Question: Say I use a font "foo" in my TeX document. 

> 
> \setmainfont{foo}
> 
> 
> 

This font is not free or not widely available on other systems.
Will my PDF document will look same in other computers where this font is not available / installed?
And if not what can I do about it to make sure that it looks same in other computers?

Answer: 
Fonts are embedded by default when you compile TeX to PDF. To be sure, you may wish to upload your PDF to a cloud, say, GoogleDrive (free) and see, how it looks there in another previewer.
I'm not an expert on PDF, but the whole idea of PDF in opposite to Word is that the viewer displays the same picture and the printer prints the same glyphs on the paper. 

Well, more or less. It obviously does not depend on the installation of fonts on the computer you use to display the PDF, but there are differences even between different viewers on the same computer. Compare the display of a PDF by Adobe's Reader and DocView (Emacs!). For an example see here: [Why do fonts look thinner in adobe and different and richer in sumatra pdf](https://tex.stackexchange.com/q/171706/4736)

And think of the properties of the printer. Or: A printout from a dot matrix printer will look completely different from a ink printer. Even laser printer produce very different output.

tl;dr: Your PDF will look quite similar, independently from installed fonts.

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### Edit:

There seems to be a misunderstanding about the license of commercial fonts. Of course you can use a fonts you bought to produce a PDF. The company which sold the fonts to you has no whatever rights on your PDF! Selling fonts includes the fair use of making a PDF. And making a PDF means that all the needed glyphs become part of the PDF. (I am a lawyer! OK, only in Germany.)

It is something different, if you embedd by using attachfile, embeddfile or whatever the \*.ttf or \*.otf file itself -- this might not be legal!

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### Edit 2:

There are companies which try to limit the use of sold fonts to a PDF with a circulation of one hundred copies, e.g. the Font Bureau in Florida (thank you, [Thérèse](https://tex.stackexchange.com/users/7883/th%C3%A9r%C3%A8se)). I read the license agreement of some of the fonts I bought and there is no limitation for a PDF, but the lincense of Linotype says:

> 
> »Embedding of the Font Software into electronic documents
>  or Internet pages is only permitted under the absolute assur-
>  ance that the recipient cannot use the Font Software to edit or
>  create a new document (read-only). It must be ensured that the
>  Font Software cannot be fully or partially extracted from said
>  documents.«
> 
> 
>