P-jacobs,
The rounding of 0.5 is only in reference to the Arial font size on the PDF side, but i'll get back to that later.
When set to imperial units (inches), text can be modified in 1/256" increments.
When on the metric units (mm) side, the refinement in mm seems a bit more random in when it elects to visually change the actual size of the text height.
Stepping back and forth between the two, my guess is that the following is occurring. The text height in mm is converted to inches and then rounded to the closest 1/256 inch increment. I worked through a few, and cannot state if it is exactly a round_up, round_down, or round_nearest function. But, i'm going to go so far as to say that 2.5mm text is most likely NOT 2.5mm. It is really 25/256" (0.097656 in), which is close to the conversion of 2.5mm (0.098425 in).
Now, as far as the 0.5 rounding that occurs to the PDF. This 2.5mm text, which is actually 25/256" (0.097656 in) will get rounded to font size 10 (or roughly 0.1000 in) in the PDF. Thus completing an upscale of the text size of about 2.4% from what it visually looked like within Revit. And "may" potentially result in why your 2.5mm example pushed past the line in the "built-in PDF Printer" example.
Some additional testing shows that built-in PDF is not adjusting the Horizontal Scaling value of the text, and keeping this at 100, whereas a PDF Printer (Bluebeam) reduces this down to 99.65.
So, to summarize:
Revit: Arial, 3/32" (0.09375 inch) text with width factor =1 will result in the following going to PDF.

Built-in PDF: Arial, 9.5 font, Horizontal Scaling = 100. This will reduce the overall length of the text.

Print to PDF (Bluebeam): Arial, 9.6 Font, Horizontal Scaling = 99.67. This matches visually what i see in Revit.

Edit:
Not every 1/256" increases the size as well. Going to have to investigate this further. My next inclination is to believe that it all relates to converting the mm or in. into font pixel (px) size and rounding to the closest pixel increment within revit. But even with this, 3/32" (0.09375) converts precisely to 9 pixel, yet i get a 9.5 to 9.6 out of it in the PDF.