First working version, no help and can only set the text rendering mode to 'Fill'

.gitignore

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+dist*/
+.ghc.environment.*

CHANGELOG.md

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+# Revision history for reveal
+
+## 0.1.0.0 -- 2019-05-18
+
+* First version. Released on an unsuspecting world.

LICENSE

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+Copyright (c) 2019, Tissevert
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Tissevert nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

reveal.cabal

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+cabal-version:       >=1.10
+-- Initial package description 'reveal.cabal' generated by 'cabal init'.
+-- For further documentation, see http://haskell.org/cabal/users-guide/
+
+name:                reveal
+version:             0.1.0.0
+synopsis:            A simple program to change the text rendering mode of a PDF
+-- description:
+homepage:            https://git.marvid.fr/Tissevert/reveal
+-- bug-reports:
+license:             BSD3
+license-file:        LICENSE
+author:              Tissevert
+maintainer:          tissevert+devel@marvid.fr
+-- copyright:
+category:            Data
+build-type:          Simple
+extra-source-files:  CHANGELOG.md
+
+executable reveal
+  main-is:             Main.hs
+  other-modules:       PDF.TextRendering
+  -- other-extensions:
+  build-depends:       base >=4.12 && <4.13
+                     , bytestring
+                     , containers
+                     , Hufflepdf
+                     , zlib
+  hs-source-dirs:      src
+  default-language:    Haskell2010

src/Main.hs

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+{-# LANGUAGE NamedFieldPuns #-}
+module Main where
+
+import Codec.Compression.Zlib (compress, decompress)
+import Data.ByteString.Lazy.Char8 (ByteString)
+import qualified Data.ByteString.Lazy.Char8 as BS (
+    length, lines, readFile, unlines, writeFile
+  )
+import qualified Data.Map as Map (insert, lookup)
+import PDF (Document(..), parseDocument, render)
+import PDF.Object (Content(..), DirectObject(..), Object(..), Name(..), Number(..))
+import PDF.TextRendering (TextRendering(..), update)
+import System.Environment (getArgs)
+import System.IO (hPutStrLn, stderr)
+
+revealFlateEncodedText :: ByteString -> ByteString
+revealFlateEncodedText = compress . revealText . decompress
+  where
+    revealText = BS.unlines . fmap fill . BS.lines
+    fill = update $ const Fill
+
+revealObject :: Object -> Object
+revealObject obj@(Stream {header, streamContent}) =
+  case Map.lookup (Name "Filter") header of
+    Just (NameObject (Name "FlateDecode")) ->
+      let newStreamContent = revealFlateEncodedText streamContent in
+      let newLength = Number . fromIntegral $ BS.length newStreamContent in
+      Stream {
+          header = Map.insert (Name "Length") (NumberObject newLength) header
+        , streamContent = newStreamContent
+      }
+    _ -> obj
+revealObject obj = obj
+
+revealContent :: Content -> Content
+revealContent content = content {
+    objects = revealObject <$> (objects content)
+  }
+
+reveal :: Document -> Document
+reveal document = document {
+    contents = revealContent <$> (contents document)
+  }
+
+main :: IO ()
+main = do
+  [inputPath, outputPath] <- getArgs
+  result <- parseDocument <$> BS.readFile inputPath
+  case result of
+    Left parseError -> hPutStrLn stderr $ show parseError
+    Right doc -> BS.writeFile outputPath . render $ reveal doc

src/PDF/TextRendering.hs

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+{-# LANGUAGE OverloadedStrings #-}
+module PDF.TextRendering (
+      TextRendering(..)
+    , update
+  ) where
+
+import Data.ByteString.Lazy.Char8 (ByteString)
+import qualified Data.ByteString.Lazy.Char8 as BS (pack, readInt, unwords, words)
+
+data TextRendering =
+    Fill
+  | Stroke
+  | FillStroke
+  | Invisible
+  | FillAddPath
+  | StrokeAddPath
+  | FillStrokeAddPath
+  | AddPath
+  deriving (Enum, Show)
+
+update :: (TextRendering -> TextRendering) -> ByteString -> ByteString
+update f = BS.unwords . changeTextRendering . BS.words
+  where
+    changeTextRendering :: [ByteString] -> [ByteString]
+    changeTextRendering l@[value, "Tr"] =
+      case BS.readInt value of
+        Just (n, _) -> [BS.pack . show . fromEnum . f $ toEnum n, "Tr"]
+        _ -> l
+    changeTextRendering l = l