Instead of attaching the full PDF file, Outlook converts and inserts only the first page of the document as a PNG image. This means that the actual PDF file is not attached, making it impossible for the recipient to download or view the complete document.
To ensure that the entire PDF file is attached correctly, you can either attach the pdf by clicking the attach icon and selecting the file, or you can use the drag&drop function.
Unfortunately, there is currently no direct fix to change how Outlook processes pasted PDFs, as it is directly a Microsoft issue at the moment.