How can I send a large pdf file (42MB) attachments by icloud ??

That's a bit excessive. The website http://convert.neevia.com/pdfcompress/ can compress files up to 5 MB in size. You may want to print to PDF single smaller pages, or print the document and scan it one page at a time to PDF, and then use the PDF compress tools to shrink it further.

There is another web tool called http://smallpdf.com. There is no file size limit and might be perfect for you. Have a look and good luck!