People often say a file "needs conversion" when the real problem is encoding or fonts. The symptoms look similar. The fixes do not.
Boxes
A hollow box or tofu square usually means the character exists but the current font has no glyph. Change the font to one with CJK coverage. See Chinese fonts and Unicode.
Question marks
A ? that replaced a character often means the text was forced through a charset that could not store it. The original character may already be gone. Conversion cannot reconstruct it.
Mojibake
Strings such as 涓浗 instead of 中国 are UTF-8 bytes read as the wrong encoding. Re-open the file with UTF-8 if you still have the source. If only the broken copy exists, automatic repair is hit-or-miss.
When conversion is the right next step
If you can read the Chinese, and you only want Simplified or Traditional forms, use the document converter or text converter. If you cannot read it, fix encoding or fonts first.