Export to PDF using Preview

I am using macOS Monterey. Before I upgraded into the newer version of the macOS, I was able to use the Preview with the function Export to PDF. Sometimes in a multi-page PDF documents, the sizes of the pages are different, some smaller, some larger. When I open the PDF document with Preview, I can further export the document into PDF using the function "Export to PDF". After this process, the pages sizes become the same, A4 or letter size, as big as I defined. (As shown in attached screenshot 1)

After I upgrade my macOS to higher version, the "Export to PDF" function is dimmed. I can no longer use this to convert my multi-page pdf documents and do the same thing. (as attached screenshot 2)

Can someone tell me how I can go back to use this function? Thanks a lot