Assuming you are referring to something which is to be printed..
2.5pt text, in any typeface, is entirely unreadable. In addition, it is most likely unprintable. At that size glyphs will plug up and become more dots than characters.
The only options you have are:
- Reduce the amount of content in order to increase the font size. I'd
suggest a minimum of 6pt type. You maybe able to get away with 5pt
type though. I'd suggest sans serif typeface with even width strokes
so that each glyph carries the relative same contrast.
- Increase the size of the document in order to retain content but
increase the type size.
Form vs. Function.
If the function of the piece is to be readable, you must change the form, be that content or document size.