Remove characters (non printable?), like open box

The original file is Excel.  There are longs lists of comments and I have inserted a hard return (alt enter) after each one to ensure there is space between each row.  The return isn't visible in Excel except as white space but if I save as to pdf or batch convert from Acrobat I then see a rectangular box.  I've tried removing 'hidden' characters using the action wizzard but it didn't do anything.  I have stripped them out before using Acrobat, I simply can't find the option/setting.  I was pretty sure it was called something like "remove non-printable characters" but I haven't found it within the software or by searching here.

What is the format of the source document and the process you are using to create the PDF?

The square boxes usually mean a font problem. You are you only using fonts that can be embedded into the PDF?

If you are using any licensed fonts, do  they allow reuse of the font?

Make sure you options for Acrobat/Reader are not causing only "local fonts" to be used.