[vox-tech] ps versus pdf - fonts and quality

[Peter Jay Salzman]

I download a lot of scientific papers off the web.  They almost always come
in two formats: postscript (ps) and portable document format (pdf).

I've noticed that, consistently, pdf looks better than ps on the screen.

Actually, pdf tends to look ORDERS OF MAGNITUDE better than ps for really old 
scientific papers that I download off the journal's archive.  The two formats
print at the same quality, but the screen quality of pdf for these old
documents (1960ish - 1995ish) is truly orders of magnitude better than ps.

For recent documents, say documents that I download off http://xxx.lanl.gov,
the screen quality is about equal (although I still think that pdf is
somewhat better than ps, but I can't be sure).  Print quality is about equal.

What's going on here?  Why do these older ps documents look awful on screen
but print at the same quality as pdf?

And why does the quality of ps vary so much?

Pete