When I edit PDF why would some text (in foreign languages) turn to strange characters

Please let me give you an example then my question would be easy:

A PDF document in English with some Arabic or Chinese text

If I edit the document mentioned above the newly saved PDF documents only retain proper English text while the characters of foreign language such as Arabic or Chinese turn in to some strange characters. I wonder why?

Does this means if the PDF contains any foreign language text those fonts must be installed on my computer otherwise the edited file could not retain the original text?

I shall be thankful to you if, someone would enlighten me please.

Thank you.

Ari