Do you go Layout > Insert > Object > Create from File > browsing to an Excel sheet file? 1) You could resave the Excel file to an .xls file if you deal with .xlsx file or vice versa. 2) Test inserting the sheet into a newly created .mxd document first.
You may get better results copying and pasting the spreadsheet cells (perhaps with a "cushion" of surrounding cells) into ArcMap. But even that approach can run into problems in exporting to pdf especially if after the paste one has to resize the table. So in those cases we create a stamp in Acrobat and use Acrobat to add it to the map pdf. Clumsy.
I discovered a solution: Insert table in data view. It will probably look messed up like the example above. If it prints like this, try this: Cut the table, and paste it into layout view. Then cut and paste it back into data view. Resize as needed. It will most likely look screwed up again as in the example, but will print fine. I tried this with 10 tables in data view that needed editing and it worked on all of them.