Hello,
sorry for the late reply. Today PDF24 opens PDF files in the PDF24 Reader. Signing directly in the Reader is not available yet: This function is in the works for PDF24 Reader and will be released with one of the upcoming versions.
Until then you can sign a PDF with the tool Sign PDF of the PDF24 Toolbox, or online: https://tools.pdf24.org/en/sign-pdf
Both save the signed PDF as a new file, which then has to be attached to the e-mail. Saving it back into the opened Outlook attachment in one step, as in your workflow (sign, save, close, send), is not possible yet. Thank you for the suggestion, we will add it to our list.