status = doc:SaveToFile(filename)
doc:SaveToFile() saves the current document to a file.
Returns #HPDF_OK on success, otherwise it returns an error code and the error handler is called.
#HPDF_INVALID_DOCUMENT - An invalid document handle is set.
#HPDF_FAILED_TO_ALLOC_MEM - Memory allocation failed.
#HPDF_FILE_IO_ERROR - An error occurred while processing file I/O.