Many backpacks are advertised as being waterproof, but the truth is that many of them struggle to be 100% effective after a while because the waterproof sealing failed.
A reliable waterproof backpack must have airtight zippers and welded seams to ensure a long-lasting waterproof seal. Moreover, pick one that’s made of durable, tear-resistant materials if you want it to last for a while. PVC is a popular choice as a waterproof backpack material, and it has proven to be quite effective.