Why are some of the letters in my PDF invisible?

I have a strange problem with one of my PDF files. In some places (usually where the text is bold), all the letters are invisible except for the "a"'s and "v"'s. When I copy the text and paste it into another program, I can see the letters displayed normally.

For example, I see:

Copying and pasting the start of the paragraph into another program, I see: "The instantaneous rate of change at".

How can I fix this?

Solution 1:

This may have to do with the embedding of restricted fonts. Try unchecking the Acrobat Preference for "Use Local Fonts" for reading, under Page Display > Rendering, as shown in the image below (or in a very similar location, depending on the version of Acrobat):

Solution 2:

I suffered from the same problem and found that just changing the font to something else then back to the original font fixed the issue.

Hope this helps.