I have a PDF file, and occasionally, there appear black filled boxes across the width of the page. The strange thing is, that when I mark the area and hit 'copy' and then paste it in a text editor, the text that's hidden inside the black boxes appear!
I tried two different PDF viewers (Adobe and SumatraPDF) and they both present the file with I also searched for online PDF repairers but they only handle corrupted PDF file which cannot be opened. I also thought about converting to PostScript and fiddle with the source code and recompile but I don't know anything about it.
So, how do I fix it?