Storage location for photos and pdf

When if at all does storing photos and pdf files on WordPress interfere with upload/navigating performance of a WordPress site?

Never.

Is the best location to store photos for a WordPress site on an external storage site like Flickr?

Depends on your needs. I would stick with using WordPress. We have clients with hundreds of gigabytes of images stored with WordPress. If you have the space I would use it.

Is it easier to manage an external storage site? Can one create separate folders for each slide show?

Folders in WordPress are organized by date year/month. A reference to where the images are stored for slideshows are saved in postmeta in the database.

What is the maximum amount of photos one should store on WordPress if this can be an issue with performance?

There is no maximum. Like I mentioned above, we have clients with hundreds of gigabytes worth of images stored with WordPress.