The fix for this issue is included in the latest version of Quick Sale (v2.7.25 or newer)
For older versions:
The best solution for this issue is to add a PayPal Pay Now button to the email text body, or if you are a subscriber, switch to one of the new invoice designs available with a subscription.
To add a "Pay Now" button to the emails:
Go to Settings > Customize Email
Add the following line to the email body: Pay with PayPal (or something similar) %PayPalLink%
When your clients open the email, the %PayPalLink% will appear as a clickable button, for example:
This will allow your clients to make a payment without the need to open the PDF invoice and it should work on all phones, tablets and computers.