as a rule a PDF will always be larger than the originating document - partly because of the way in which fonts are embedded in the PDF file. How much larger depends on the file content - could 10%, could be 500%. Also different PDF writing programs have different results. All you can do is try several and see.
However, despite that, distributing in PDF makes a lot of sense, even if only to hinder others from tampering with the document post-distribution