Why are some people unable to open invoice using the link
Are you able to give more detail at all?
Can they open the link at all? Are you emailing it to them yourself, or using Quick File to send the email? Are you trying to download a PDF, or just open the link?
Hopefully this will help pin point any issues, and able to help further 
I am sending link by e mail but through Quick File. Its just the link I am sending but then have had to send as PDF with them not being able open the link.
Are there any errors being shown do you know?
One thing I have found with one of my clients, is that the invoice will load but without any of the buttons or graphics, mainly because they’re behind a corporate filter
No error code as far as I am aware however one client sent this
I have carried out a trouble shooting program on the link and the results say this device or source is not responding: https://kidzrfit.quickfile.co.uk/