set "p=path to directory with fonts with spaces in path"
for %%f in ("%p%\*.*") do tx -pdf -dd "%p%" -a "%%f"

I wonder, why this did not work:

for %%f in ("path to directory with fonts with spaces in path\*.*") do tx -pdf -dd "%%~dpf" -a "%%f"

The dd-option does not seem to like a backslash in the end of a path. You may also have a view in the thread Easy way to generate a Glyph/Character.