Convert string to multiline text
First Impressions 
I haven't gone through the functionality in detail but a few things strike me immediately.

Why explicitly include StringSplitOptions.None. They default value is none.

IsNullEmptyOrWhiteSpace() seems to be a (not included) extension method for string that performs the same function as String.IsNullOrWhiteSpace.  Does it do something else as well?

The check for an empty 'line' inside the loop

if (line.IsNullEmptyOrWhiteSpace())
        continue;


should be redundant as we have previously filtered the lines

var filterLines = lines.Where(line =&gt; !line.IsNullEmptyOrWhiteSpace());


I would need to do up some performance checks to be sure but I reckon that the use of the StringBuilder (presumably for performance reasons) will be negated by the calls to line.SubString(). 

I can't tell for certain what the requirements are  - I would guess

Max line Length of n
Do not split words
Do not end on a space
Do not start on a space


but it look like a linear progression through the string, with some back tracking to deal with split words/spaces, might work and appending the individual chars might give you better performance.

The final return should be

return multiLine.ToString();


there is no need to create another string and place the result of multiLine.ToString() inside it and then return the anonymous string.