If the site is defined  as "http://portal", then you may be out of luck
there. If so, a workaround for your workflow  might be to use another
variable like Server Relative URL or Path, and append the domain name to the
beginning as part of a dynamic string.

You can change the default  URL. Basically, just swap the URLs in 'Edit Publich
URLs' and that should change the workflow  URL. At least, this worked pre-SP1.

I tried to concatnatate with Path variable and Name variable. It is
working fine, but if any file name having spaces (i.e. Alternate
Mapping.docx), it took only http://myportal.com/documents1.Alternate
Mapping

Can we resolve this ?

Benjamin's solution would be preferable if you can change this in Central
Admin.

I already tried, but no luck. Any other advise.

I had the same problem  and did the following to get around it.

I created a list that contain parameters I wanted to use for all of my
SP workflows - Parameter Name and Parameter Value are the columns I
used. For instance, I created a list item with the Parameter Name:
SiteURL and Parameter Value: http://mysite/sites/sitename. I set a
variable in my SP workflow  to the value of my parameter and then used
the Build Dynamic String to create the entire url.

But it will if any document name contain space in their file name. For ex if
file name is "change password.txt", it will only add "change" in your url  like
http://portal/spdocs/change . It should be
http://portal/spdocs/change%20password.txt or http://portal/spdocs/change
password.txt.