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Here’s a regular in-line equation [latex]\vec F_{12} = \frac{1}{{4\pi \varepsilon _0 }}\frac{{q_1 q_2}}{{r_{12}^2 }}\hat r_{12}[/latex]

Here’s the way they had a centred equation (not using HTML) on its own line. This is ideal, but it isn’t supported in PDF exports:
\begin{equation}
\vec F_{12} = \frac{1}{{4\pi \varepsilon _0 }}\frac{{q_1 q_2}}{{r_{12}^2 }}\hat r_{12} \label{Coulomb1}
\end{equation}

Here’s the same equation centred with HTML in a styled <p> tag with font-size 110% (PDF-compatible):

[latex]\vec F_{12} = \frac{1}{{4\pi \varepsilon _0 }}\frac{{q_1 q_2}}{{r_{12}^2 }}\hat r_{12} \label{Coulomb2}[/latex]

Here’s the same equation centred with HTML in a styled <p> tag with font-size 113% (PDF-compatible):

[latex]\vec F_{12} = \frac{1}{{4\pi \varepsilon _0 }}\frac{{q_1 q_2}}{{r_{12}^2 }}\hat r_{12} \label{Coulomb3}[/latex]

Now in a styled <p> tag with \large LaTeX command (PDF-compatible):

[latex]\large \vec F_{12} = \frac{1}{{4\pi \varepsilon _0 }}\frac{{q_1 q_2}}{{r_{12}^2 }}\hat r_{12} \label{Coulomb4}[/latex]

For reference, here’s an equation that is centred with HTML, PDF-compatible, but not specifying size — it was coming out smaller:

[latex]\vec F_{12} = \frac{1}{{4\pi \varepsilon _0 }}\frac{{q_1 q_2}}{{r_{12}^2 }}\hat r_{12} \label{Coulomb5}[/latex]