I prefer to use excel to make legends as I find it quicker than using ArcMap. Occasionally in some projects when I copy these in they appear fine in ArcMap, but some aspects don't export.
ArcMap layout view:
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If you are importing that as a picture, then Export in JPEG.
ArcGIS wants to recognize the features its exporting to PDF and layer, whereas JPEG is just an image. I've had that happen a bunch of times. You can change what font/format you use in Excel, but I don't know what a better option is, and that may help. Then it's up to you how you want to convert to PDF if you really need it that format. Alternatively you can try to import that graphic from excel as a different image type.