Pocket PA is a powerful and modern 'web based browser' application and is not a native 'app' so there is nothing to download at the Apple iTunes or Google Play stores.
Pocket PA is also known as 'progressive web app' (PWA) so you can instead download Pocket PA to your phone or tablet by following this link using Safari browser
That also means you do not need any storage space on your device - just open any web browser and ensure you have Wifi or 4G.
Go to any Web browsers, examples include:
Apple Safari
Google Chrome
Mozilla
Opera
Microsoft internet explorer
Type the 'URL' (Pocket PA name) into your browser at the top bar - so this can be either
the Pocket PA website name PocketPA.com and then go to the top right to Log in to your Pocket PA account with your username and password
OR
app.pocketpa.com/login and log in to your Pocket PA account with your username and password
Select the Remember Me tick box for speed of accessing your account and save the login page to your Home screen for quick and easy access again and again - click here to see how to create your shortcut button.