Sending a PDF as an attachment triggered by Flow through Desk

I currently have a Flow that is triggered by a webhook and retrieves both the link to a pdf and the pdf itself using invokeUrl. I am trying to send both the link and the pdf (as an attachment) by the "Zoho Desk - Send email reply" function. I have managed to send the link in the Content field of this function, embedded in HTML, but currently have not managed to attach the pdf. This function has an Attachment IDs field, but I don't have an attachment ID for the pdf, and there is no Add attachment to Ticket function in Flow. I have a function returning the pdf file as ${invoice}, I also have the webhook returning the download link ${webhookTrigger.payload.DownloadLink}. I have tried and object, embed and iframe with both the invoice and the download link but neither have worked. Thanks in advance for your help.