Wny can I not import some .pdf files into IMovie

iMovie supports PDF files, so you should be able to import them. You didn't mention where the PDF file was created, and whether it was a single file or contained multiple files, or whether the files were photos or word documents or some other kind of document.

In any event, try opening a PDF file that won't import and resaving it as a PDF. Then see if the resaved one will import.

If still no luck, try deleting preferences. To delete preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can import a PDF file. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

-- Rich