BAK files are generally just renamed backup copies of the original file, so if your file was PDF, you can simply rename it back and open using PDF Reader. If that is not possible, the BAK file is not a PDF and there is no way how to convert it.
This conversion entry was verified by us and conversion from bak to pdf was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.