There's another possible complication beyond those enumerated in the excellent Re: Convert PDF file into HTML file.

Some .pdf are created by scanning text_on_paper ¹. The intermediate is an image, not unlike a .png, .jpg or .bmp. The resultant .pdf contains a picture of the text, not the ASCII or UTF or Kanji characters, per se.

And that, TTBOMK, leaves only the OCR option for retrieving the text as text.

Update: Addition below, for clarity:

^1.   This is typical, for example, of low-cost home "MFC" and "all-in-one" printer-scanner-copiers and of offices with limited, low-level IT knowledge and support and is effected by use of the multi-function copiers now commonly replacing single-purpose Xerograpic copiers.