With LO 4.2 (also occurrs with 5.0), I imported a pdf under Mac OSX 10.11.3. The words with “I” before “f” were misspelled, as can be seen by the red lines from the spell check. Preview shows the correctly spelled words in the document. Unfortunately I cannot attach the screenshot because I don’t have enough points. I tried to search for an existing bug, but the process is so cumbersome.
This started happening after I upgraded to OSX 10.11.3. The behavior was fine in OSX 10.9. I have opened the document many times.