Hi , @HaukeSan
According to your description, you want to use the sendEmailV2 to send the pdf file in your sharePoint list .
I test it in my side and this is my list :

And we can use this code in Power Apps:
Office365Outlook.SendEmailV2("AniyaZhang@xxxxxxxxx.onmicrosoft.com","email with attachment" , "here is your pdfs",{Attachments:ForAll( Gallery2.Selected.Attachments,{Name:ThisRecord.DisplayName,ContentBytes:ThisRecord.Value,'@odata.type':""} ) })
And the Gallery2.Selected Attachments in my side is like this:

Best Regards,
Yueyun Zhang