Mobi Conversion

Hi
I recently bought some ebooks on a disc, they were all in mobi format but I have obviously pressed a wrong key somewhere and now all the books are in PDF format.

Does anyone know how I can put them back to mobi format without using Calibre please?

I bought the books because I have just bought my partner a Kindle touch for xmas and I wanted him to have some books to read straight away .

Many thanks :)