.pdf versus .pdf2 confusion

Hi,

I created a promo single page using InDesign. I used its export feature to make it into a PDF. Here on my 10.6.8 machine, the filename extension is .pdf.

I emailed it to the client (who uses OS X v10.7) who is sending it on to his target audience. If he drags it into an email in Mail he can see the image of the PDF in his email, and the recipients have no difficulty opening it no matter what system they may use. But if he selects the PDF from the Choose File dialog in Mail, it appears in his email before sending as an icon, and some recipients have difficulty opening it. I asked him to repeat that process to send it back to me. I received it with a filename extension of pdf2. I too couldn't open it, but when I changed the filename, removing the 2 from the end of it, it opened fine.

So now the hard part. Why would a different method of getting the file into an email for sending create a different result? And how does that different method cause the filename extension to change?

Thanks for any hints you can offer.

Rob