We downloaded the latest version of OE on Windows 7 Pro, 32 bit and everything is working as promised... a big thank you.....

The only thing not working is when we try to attach a pdf file to an email and send it... we use Acrobat 8.1 Pro... and here's the error we receive when we choose attach to email from Acrobat:

"The procedure entry point Stream Substring match could not be located in the dynamic link library MSOERT2.dll"

Does anyone have an idea how to resolve this issue ?

You are trying to attach the pdf to mail directly from acrobat , this cant be done because as i write on oe download page, windows 7 uses different mapimail (the service that adds attachment) and it is incompatible with OE, that's why i have made the oe 'SendTo' app.
So i suggest you to save first your pdf then use 'sendto' to attach it to a new email (right click>Send to> outlook express mail recipient).
Or add it manually.