The font size within the PDF should be the same as in HTML provided that the HTML width is lesser or equal to the PDF page printable area width.
If the HTML width is greater than the printable area width the HTML content is scaled down to fit the printable area width which leads to smaller font sizes.
Here are some examples you may want to try out. The examples assume the A4 paper (8.3''x11.7'') format with 0.2'' horizontal margins. The printable area width is 8.3''-(2x0.2'') = 7.9 inches which is 568.8 points.
No width specified, the font size within PDF is 12pt.
12pt font - no width specified
The HTML width is the same as the printable area width. The font size within PDF is 12pt.
<div style="font-size:12pt; width:568.8pt">
12pt font - HTML width == printable area width
The HTML width is 2x the printable area width. The font size is scaled down by a half.
<div style="font-size:12pt; width:1137pt">
12pt font - scaled down to 6 pt font
Hope this helps.