What App is best for creating Lead Magnets

By Lead Magnet, I assume you mean some downloadable item that will entice people to give you their personal contact information.  Lead generation tactics like this rarely work unless you verify the user's email address BEFORE you send them the download link. 

That said, use Photoshop for raster images. 

https://www.adobe.com/products/photoshop.html

Use Illustrator to create vector graphics.

https://www.adobe.com/products/illustrator.html

Use InDesign to layout your e-book, brochure or pamphlet with text & graphics.

https://www.adobe.com/products/indesign.html

Export book to PDF with Acrobat Pro DC  -- the complete solution for creating and saving PDF.
https://acrobat.adobe.com/us/en/acrobat/acrobat-pro.html

Ask your web developer how to do the rest.

Hope that helps.