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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:

• Nuget: http://www.nuget.org/packages/RingCentralSDK/
• GitHub: https://github.com/ringcentral/ringcentral-csharp/tree/develop

Get the package from Nuget and follow the fax instructions under "Send Fax" on the GitHub link.

Ref: https://forums.developers.ringcentral.com/questions/226/how-to-attach-pdf-to-a-fax-message.html