When you export to a different file format, particularly PDF or html, and then back again you should expect problems with whitespace no matter what viewer you use. For instance web browsers (html) do not support tab characters at all for alignment (you are expected to use tables in html). PDF also translates various characters differently, for instance tabs can become spaces.
Hence if you want to maintain whitespace exactly save as a raw text file (.bas or .txt), do not try to use pdf or html.