I noticed it too but since the pages are readable, I didn't comment on it. It's the same for my picture on the gallery, it's all pixelised although the file or my picture isn't.
The files you sent were at the right resolution but I have a question.
When you worked on those pages, did you use the same file? Or did you paste/copy the pages into a different file to create the TIFF version?
Although the TIFF file was at 300 DPI, maybe it wasn't the case of your original file. That's a possibility to explain the pixelisation, although it doesn't explain the pixelisation of my drawing.