When one "drops" a vector PDF into an open Illustrator document the external file is then linked within the open document. i.e. "Dropping" creates a linked reference in the Illustrator document.
It's the same as if using
File > Place from the menu.
With linked vector files, if the original file changes the internal link in other documents will need to be updated, and Illustrator will present
an alert stating as much. Open Illustrator documents containing links may auto-update linked content when the content changes.
Linked vector files can't be edited. In spite of containing vector content, they are placed as one, singular, object - very similar to linking a raster image. As opposed to editable individual elements in the vector file.
Only embedded vector content can be edited. Embedding a vector file removes any linking and places the actual file contents in the open document being worked on. Embedded vector files contain no reference to any external content. Embedding the vector file is essentially the same thing as copy/pasting the content from one file to another - content is embedded as live, individual, vector objects.
If you wish to edit the contents of a linked vector file, edit the original file.
If you wish to remove the link reference and have all the content editable in the file you've dropped the external document into, you must embed the file, not link to it.
To embed an existing linked vector file ....
- Find the file link in the Links Panel (
Window > Links).
- Highlight the link in Links Panel
- from the Links Panel Menu choose