Did you try zipping the file?
If you have acrobat pro, you can adjust the quality better than you can from the pdf settings of photoshop.
Another option is to send the pdf to a server and send the link via email.
Also when creating a document for this purpose, your better off using as much vector graphics as possible ver. normal raster tools. As the file size is much, much smaller. So Text tool, pen tool, shapes, paths, etc are your friend for this type of work. Not to mention you don't have to worry about pixel size, or the ppi, just the page size. Because vectors are nothing more than mathmatics and are therefore resolution independant.
Like Silkrooster said, vectors (and type) can be useful.
Is the psd CMYK or RGB?
What settings do you use when creating the pdf?
Could you post a screenshot?
Expecting any considerable quality when compressing a 2550px x 3000px image to under 100kB might be a bit ill-advised …
You would not likely be able to get anything more than a blank, white image of that size at 100kb.
Not even Acrobat Pro will help here.
This task seems silly. What web sites require PDF files be less than 100kb? Some ordinary web graphics (PNG/GIF/JPG) might exceed that size.