A common issue for some browser is to automatically block all popups from websites, which is generally a good thing but can also stop your Quotes, Invoices and other PDF files in MyConstruct from being opened in a new window.
MyConstruct will alert you to this setting when it happens (below):
To solve this you simply have to enable popups for MyConstruct (don't worry this will only enable popups for the MyConstruct website, not the whole internet)
For Mozilla Firefox:
For Edge / Internet Explorer:
Be sure to use the 'enable' option for each tutorial above.