Our send letters and send fax endpoints accept a public URL to a PDF file.
If you prefer to not supply a public URL, there's another option: You can pass a file object to our 'uploads' endpoint first, which returns a URL that can be used with any of our send endpoints. The uploads endpoint returns a URL which is public by default, but can be changed to private here: https://dashboard.clicksend.com/#/account/settings/security. If you set it to Private, only we will be able to access that URL. It will also be destroyed within 10 mins.
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