Please guide me on how we can attach PDF doc to a FAX message . Also, would like to know the attachment limit
You can send fax with PDF, all you need is to create a multipart/mixed request and add each PDF as an attachment. You can either send the raw bytes or base 64 encode the file.
There are many way to do this with C# is to use (alpha) SDK which is available on Nuget and GitHub:
Get the package from Nuget and follow the fax instructions under "Send Fax" on the GitHub link.