You can insert a PDF into Google Slides by converting it into images or creating a link to the same. This post covers both methods in detail.

If you want to lock images or objects in Google Slides, you can Group them, use Master Slide feature or use the photo as a Background.

Google Slides are an easy way to create and share slideshow presentations. You can even draw free-form on Slides when you're creating them. Here's how it works.