How can I store photos on my iphone in a jpg or pdf format

Do you mean when you take photos with your phone, or sync photos to your phone? To sync just use iCloud Sync; it won’t change the format of the images.

If you want the photos you take to be JPG go to Settings/Camera and change format to Most Compatible.

Most Compatible won’t change the format of existing photos, but all new photos you take will be JPG.