A counterpoint to the path vs text argument: yes, saving as path will preserve the look of the document, but so would exporting the whole thing as a PNG (I have gotten emails like this, and it is just slightly irksome). If you want semantic meaning to be delivered with the document (that is, if you want people to be able to select text to copy/paste or if you want machines to be able to tell roughly what is in the document), then text is the way to go.
Another consideration is that converting to paths may not actually help you reduce size, since all the vector curves in the letters have to be specified for each letter shape in the file, rather than just saying "This is a Q, look in the Q slot of the font file for the vectors to render it." Although I have not done so, I know it is possible to embed fonts inside PDF files, and although it would increase your file size considerably, it might still be smaller than converting to paths depending on the size of your source text. Alternately, you could supply the font(s) alongside the PDF file, to be installed by the end user.
In the end, minimizing vector file size is a technical art, in this case complexified by the conversion from SVG to PostScript, which PDF uses. The ifs and hows of doing it depend a lot upon your use case - who are you sending these files to, how many people, what is the mode of distribution, and can you correspond easily with the recipients to provide support?