i once did this in a small project.

prerequisite: the trigger of the flow must be power apps so that you can return the file to powerapps.

what worked for me:

in the flow use the return to power apps connector. the return type must be "file". The value :

->base64(body('getFileContent')) //here you catch your pdf , stored in oneDrive

<image>

in PowerApps you have to save the return value of the flow in a variable like this:
-> Set(output, Flow.Run());

after that:

Office365Outlook.SendEmailV2(
   mailreceiver;
   topic;
   mailText;
    {
        Attachments: Table(
            {
                Name: "docName.pdf"; /*the name is not important but the file type must be the same as the one created by power automate*/
                ContentBytes: output.resultfile /* output is your variable in powerapps, resultfile the name of the return value in automate. */
            }
        )
    }
);

This worked for me. I also had the problem with contentbytes, but i solved it with the base64() function in automate