Email will not attach my PDF document

I am using an Internet based email provider called Horde.  On my computer (Windows 7) and another station in my office (Windows XP) when I try to attach PDF documents to an email message it shows the attachment was successful but the icon next to the attachment looks like this:

1. cox bills.pdf  (text/plain)   Size: 1,478.13 KB

When I send the email my recipient can not open the PDF document.  However, when using the two other stations in my office (both Windows XP) it shows the attachment was successful but the icon next to the attachment looks like this:

  1. cox bills.pdf  (text/plain)   Size: 1,478.13 KB

All I can tell is that the icon is different...  Why are two computers failing to correctly attach the PDF's?  Any other file type is successful, IE DOC, JPG, TIF, XLS

Any help or suggestions is greatly appreciated.