Large .pdf files

Made a two page document of family history - text and photographs - with the to me new DTP software Scribus. Saved it as pdf to be able to send it to my niece and nephews. But when I attached the pdf document to the email, it turned out that it was too large - 12 Mb even though the original document was 40 Kb only. 

Oh, well, I captured the document from the screen with my faithful ScreenHunter and sent it away.  

But what happened in the conversion to pdf, I wonder.