FlipViewer works with vectorized fonts for optimal viewing quality. The FlipViewer Creator software has to extract the fonts from the PDF for the vectorization; these are present not error-free, nor the optimum result can be achieved. Usually the PDF itself looks completely error-free, so that's ansich no indication of error. Whether there are problems with the font integration, you can find out by opening the PDF in Adobe Acrobat Pro, then "File -> Properties". Then you will find in one of the tabs the following view: PDF properties
The erroneous writings are marked by the light gray "F" icon. We then recommend the PDF again to be generated from prepress.