Add clipboard format "Portable Document Format"
Inkscape copies to the clipboard in a variety of formats:
Among those is application/pdf.
Many other vector editors register the PDF on the clipboard with the name Portable Document Format, for example Affinity Designer:
Some programs actually expect the PDF format to be called Portable Document Format and will not allow to paste application/pdf.
If Inkscape would just store the same contents also under the name Portable Document Format there would be better interoperability.

