LaTeX portability comes in multiple ways:
1.An actual LaTeX file is merely a text file, which is just about the most portable format in computing.
2.The LaTeX system that processes the text file and produces the finished document has been implemented on just about every mainstream platform you care to mention.
3.The default output file format for LaTeX is DVI (which stands for device independent). This was around well before PDF was dreamed up and the high quality files can be viewed via software viewers or printed out. DVI is an open standard, so once again, readers are extremely portable and exist on most operating systems. Admittedly, DVI is hardly ubiquitous and nowadays it's often bypassed in favour for PDF (or it's very simple to convert to other formats like PS or HTML)