problem with copying and pasting Photoshop PDF into Photoshop PDF

I'm having difficulties simply copying and pasting images, mostly PDFs, from one Photoshop file into another Photoshop file.

By @birckcmi

A little more clarification here…do you mean that the PDFs:

A. Were originally PDF files, but were opened by Photoshop and rasterized by the Import PDF dialog box. In this case, they are no longer PDF files, but Photoshop documents and should work as Photoshop documents.

B. Are PDF files imported into an existing Photoshop document using the File > Place commands or drag and drop, so that they are now PDF Smart Object layers in that Photoshop document.

Copy and paste should work either way, but if it’s one or the other it might help us figure out where it’s going wrong.

Another thing that might narrow it down…if you are pasting using the Command-V keyboard shortcut, what does it look like if you paste using the Edit menu instead? For example, after copying and switching to the other document, are all applicable Paste commands available, or are any unavailable? (see the image below) This might tell us whether the reason it can’t paste is because of something related to what’s active in the destination document.

For example, in the image below, after using Edit > Copy on a marquee (pixel) selection, normally almost all of the Edit > Paste and Paste Special commands should be available. (It’s normal for Paste Without Formatting to be unavailable because I selected pixels, and Paste Without Formatting applies only to copied text characters.)

Also are you copying a marquee selection, or an entire layer selected in the Layers panel?

And does it work to instead drag and drop the selection between documents?

Can you paste copied content into another application? Perhaps something is preventing your clipboard from accepting content?