You can export iBooks Author document format to common PDF using the export function within the program.
Convert iBooks Author document file to Adobe Portable document format.
We found one software available in the database for this conversion.
Apple macOS / Mac OS X software - convert book to pdf on OS X
Apple's free authoring tool for making great looking e-books for iPads
Convert from book file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.