Add the font-family of Font Awesome to the list so it get used for the icon:
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.11.0/css/all.css" >
Before<br>
<i style="font-family: Verdana, Geneva, Tahoma, sans-serif" class="fa fa-plus-circle" >
Invoice number: </i>
<br>
After<br>
<i style="font-family: Verdana, Geneva, Tahoma, sans-serif, 'Font Awesome 5 Free'" class="fa fa-plus-circle" >
Invoice number: </i>
Related: Font Awesome 5 - Choosing the correct font-family in CSS pseudo-elements
But better avoid using text inside the icon element to avoid such issue and also avoid inheriting the font-weight
of the icon:
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.11.0/css/all.css" >
<i class="fa fa-plus-circle" ></i><span style="font-family: Verdana, Geneva, Tahoma, sans-serif" >Invoice number: </span>
If you try to change the font-weight
you will break the icon because that version may be a Pro one:
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.11.0/css/all.css" >
<br>
<i style="font-family: Verdana, Geneva, Tahoma, sans-serif, 'Font Awesome 5 Free'" class="fa fa-plus-circle" >
Invoice number: </i>
<br>
<i style="font-weight:400;font-family: Verdana, Geneva, Tahoma, sans-serif, 'Font Awesome 5 Free'" class="fa fa-plus-circle" >
Invoice number: </i>
Related: Font Awesome 5 on pseudo elements shows square instead of icon