Why do invoices which are linked to an estimate appear with smaller & grey font compared to invoices created without an estimate? They are using the same template.

Great to have you here in the Community, @577525. 

Let's perform some basic troubleshooting steps so you'll be able to use the correct template for an invoice linked with an estimate. 

We haven’t received any reported issues about QuickBooks unable to use the default template when converting an estimate to your invoice. 

To help fix this, let’s first check your Adobe setting and make sure it's updated. This way, it helps resolve the template discrepancies, such as smaller and grey fonts. For more details, you can check out this helpful article: Update Adobe Reader/Acrobat. 

Then, you can use private browsing or incognito mode. It's a best way to help isolate the issue since it doesn’t store data in the cache. You can also verify to see if this is a browser-related issue or not.

If you're using Internet Explorer and Mozilla Firefox, you can press Ctrl + Shift + P to open a private browser. For Safari, press Command + Shift +N. You can press Ctrl + Shift + N if you're using Google Chrome. 

If it works, you can start clearing your regular browser cache since it helps to get a clean slate. For more information, see this article: How Do I Clear Browser Cache and Temporary Internet Files. 

If the private browser doesn't work, I'd suggest using another compatible browser. 

After that, you'll need to sign back in to your QBO account. Then, let's check if you're able to see the correct template for an invoice linked with an estimate. 

This will get you going, @577525. I’m always here to help if there's anything else you need.