My clients wants to access their photos on the phone/tablet.

Pixieset collections are designed to be responsive. When you upload your images we automatically resize them to multiple sizes so they will display correctly and efficiently depending on the viewing screen size and resolution. 

Your clients will not need to download an App, they can simply access the link on the mobile browser! The images in the collection will render for that specific device. Your client can also click "Add to Homescreen" on their iPhone/iPad or Android devices for quicker access.

Add to Home Screen:
Apple: http://support.apple.com/kb/TI42
Android: https://developer.chrome.com/multidevice/android/installtohomescreen

Related:
How do I add a gallery as a mobile web app?
How do I download my images to my mobile device?

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