Can we see a code that is responsible for sending a PDF file via e-mail? Your problem might be caused by not released memory stream. If you're using an Attachment class then you should do like the following:
using (Attachment data = new Attachment("document.pdf", MediaTypeNames.Application.Octet))
{
}
After calling the Send method make a call to Dispose of your mail message object:
objSMTPClient.Send(objMailMsg);
objMailMsg.Dispose();