There is no definite solution but I found 2 solutions that works most of the time.

In python https://github.com/JoshData/pdf-redactor gives good results. Here is the example code:

# Redact things that look like social security numbers, replacing the
# text with X's.
options.content_filters = [
        # First convert all dash-like characters to dashes.
        (
                re.compile(u"Tom Xavier"),
                lambda m : "XXXXXXX"
        ),

        # Then do an actual SSL regex.
        # See https://github.com/opendata/SSN-Redaction for why this regex is complicated.
        (
                re.compile(r"(?<!\d)(?!666|000|9\d{2})([OoIli0-9]{3})([\s-]?)(?!00)([OoIli0-9]{2})\2(?!0{4})([OoIli0-9]{4})(?!\d)"),
                lambda m : "XXX-XX-XXXX"
        ),
]

# Perform the redaction using PDF on standard input and writing to standard output.
pdf_redactor.redactor(options)

Full Example can be found here

In ruby https://github.com/gettalong/hexapdf works for black out text. Example code:

require 'hexapdf'

class ShowTextProcessor < HexaPDF::Content::Processor

  def initialize(page, to_hide_arr)
    super()
    @canvas = page.canvas(type: :overlay)
    @to_hide_arr = to_hide_arr
  end

  def show_text(str)
    boxes = decode_text_with_positioning(str)
    return if boxes.string.empty?
    if @to_hide_arr.include? boxes.string
        @canvas.stroke_color(0, 0 , 0)

        boxes.each do |box|
          x, y = *box.lower_left
          tx, ty = *box.upper_right
          @canvas.rectangle(x, y, tx - x, ty - y).fill
        end
    end

  end
  alias :show_text_with_positioning :show_text

end

file_name = ARGV[0]
strings_to_black = ARGV[1].split("|")

doc = HexaPDF::Document.open(file_name)
puts "Blacken strings [#{strings_to_black}], inside [#{file_name}]."
doc.pages.each.with_index do |page, index|
  processor = ShowTextProcessor.new(page, strings_to_black)
  page.process_contents(processor)
end

new_file_name = "#{file_name.split('.').first}_updated.pdf"
doc.write(new_file_name, optimize: true)

puts "Writing updated file [#{new_file_name}]."

In this you can black out text on select text will be visible.