Can't modify my PDF file

If it has been saved from another application, then it's better to edit the source file in that app. But if the PDF is all you have, then you will need to embed the "pages" (in case it has multiple) and then also release clipping masks and group. Use the Layers panel to inspect the file.

Did you save it with the option to Preserve Illustrator Editing Capabilities turned on?

It is recommended to save as .ai if you want to continue editing the file.