Text turns into Squares with X's

[Myra5FD2]

I work at a company that is using Adobe Acrobat Pro DC, and I download PDF's and combine them and will add text. Lately when I send it to the next person when they open it up (I don't think they have Adobe Acrobat Pro DC, I believe they just have Adobe Reader) the letters and numbers I added at random will turn into Square's with X's in the middle. Online everywhere it says to set it up so that the Font will imbed, but I can't find any updated information on how to do that in Adobe Acrobat Pro DC and my IT says I just need to change the font. 

[Karl Heinz Kremer]

If you are combining documents, you are very likely running into this problem: 

http://khkonsulting.com/2015/02/missing-characters-merging-inserting-pdf-files-potential-workaround/

I wrote this more than five years ago, and I have not tested the workaround with a recent version of Acrobat, but all the funcgtionality should still be there.