The output from pdftohtml for one of our pdfs contained the byte sequences:

  \355\240\265\355\261\203 and \355\240\265\355\261\210

They seem to correspond to UTF-8 encoded surrogates for U0001d443 and U0001d448 (MATHEMATICAL ITALIC CAPITAL P and MATHEMATICAL ITALIC CAPITAL U). The proper UTF-8 encoding for these characters is \360\235\222\203 and \360\235\221\210.

Many UTF-8 decoders and validators follow RFC 3629 and will reject UTF-8 encoded surrogates. From RFC 3629 section 3:

  The definition of UTF-8 prohibits encoding character numbers between U+D800
  and U+DFFF, which are reserved for use with the UTF-16 encoding form (as
  surrogate pairs) and do not directly represent characters. When encoding in
  UTF-8 from UTF-16 data, it is necessary to first decode the UTF-16 data to
  obtain character numbers, which are then encoded in UTF-8 as described above.

and:

  Implementations of the decoding algorithm above MUST protect against decoding
  invalid sequences. For instance, a naive implementation may decode the overlong
  UTF-8 sequence C0 80 into the character U+0000, or the surrogate pair ED A1 8C
  ED BE B4 into U+233B4. Decoding invalid sequences may have security
  consequences or cause other problems. See Security Considerations (Section 10)
  below.