siPDF files

I have some ebook files with the siPDF extension and want to add the ebooks to my Calibre collection. The siPDF files are about 1000 times the the size of a simple PDF file of a similar ebook and is handled so slowly that I end up cancelling the read.

Will Calibre in fact read an siPDF book if I just wait long enough?

If not, is there any software that will convert the siPDF book to some other format that Calibre will handle readily?

Thanks for any suggestions.