The Helvetica from Adobe fonts are not ttf so it's likely that it will be very ugly. You may replace them with something similar to Helvetica like fonts-freefont-ttf
package, or use sans-serif, as described in the Debian FAQ:
nano -w ~/.fonts.conf
This will create the fonts file, now just put the following:
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
<!-- Helvetica is a non true type font, and will look bad. This
replaces it with whatever is the default sans-serif font -->
<match target="pattern" name="family" >
<test name="family" qual="any" >
<string>Helvetica</string>
</test>
<edit mode="assign" name="family" >
<string>sans-serif</string>
</edit>
</match>
<dir>~/.fonts</dir>
</fontconfig>
This will replace any finding to the helvetica font for sans-serif.