I had the exact same issue. The following solution worked for me:
1. Go to the Steam downloading location where the problem PDFs are: likely to be something like C:\Program Files\Steam\steamapps\downloading\223750\Doc
2. Make a note of the names of the PDFs (For me the broken files were: DCS User Manual EN 2020.pdf and DCS User Manual RU 2020.pdf)
3. Delete those PDFs (again, make sure you are in 'downloading' directory
4. Copy in some working PDFs (can be anything but I just used a random DCS manual in the main install directory)
5. Rename your new PDFs as the names you made a note of in step 2
6. Continue the Steam download, don't restart (otherwise you'll have to do it again).
Have a cup of tea and a biscuit to celebrate.
