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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:

PDF with invisible letters

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

How can I fix this?

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Have you tried opening it in another pdf reader (especially in Acrobat Reader). Especially some mathematical expressions are not supported by all implementations. – Tim Oct 11 '13 at 7:51
same reason as this… – Haicim Oct 11 '13 at 12:21

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):

enter image description here

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That didn't work – mowwwalker Oct 14 '13 at 0:48
Sorry to hear that. What you're left with is going to be examining the fonts used in the document, and then either installing what's missing, or forcing a substitution. There's a procedure here: essentially "Advanced Editing > Touch-up Text tool > right-click > Properties" and replacing with a more standard font. – Debra Oct 14 '13 at 1:02

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