There are 2 things going on here:
1. The resolution of the raster png that will be embedded in the pdf (pdf doesn't support EMF pictures which are a Windows-only file format, so we need to raster them)
You can change that by writing something like:
FlexCelConfig.DpiForImages = 1200;
Somewhere in your code. This is a static variable so it will affect all images in the app.
Just watch out for the generated pdf sizes. They probably won't grow much, but this could depend on the type of images you have.
2. Sadly GDI+ doesn't antialias the text when converting EMF files to pdf, and we can't use any unmanaged API for this. So even when you up the resolution with the code in point 1., text won't be completely smooth. There is no way we know we can fix this without going unmanaged (and we really want to keep FlexCel a fully managed dll). But I think the results with higher dpi might be good enough.