Books is suddenly opening all my PDF's. How to stop??

Thank you for clarifying Mark_e, we apologize for missing that detail.

Where are the PDF files originating from when you AirDrop them to your iPhone?

If you test AirDrop a PDF file from another device, does the same thing occur?

Have you restarted both devices?

Are they both on the same network, or signed in with the same Apple ID?

We'll work from there.

Take care.

Hi there Mark_e,

Welcome to Apple Support Communities. We understand you'd like to select which program to open your PDF files and have some steps for that. It sounds like the Books app was selected to always open PDF files. The best way to change that is as follows:

1. Find any PDF file to use.
2. Right click (or control-click) on that PDF file.
3. Choose 'Get Info'.
4. Find and expand the section for 'Open with'.
5. You can then select which other application you'd like it to open with. Make sure you choose "Change All..." to include more than just the document you have selected.

Best regards.