Text/character loss when inserting PDF files into a PDF file

Help! I'm trying to insert multiple PDFs inside a larger, bookmarked and divider-separated PDF; sometimes, mostly with files derived from Excel spreadsheets, it inserts the PDF with Xes, boxes, and gibberish filling the charts instead of showing the original numbers/characters. Is there any way to fix this? So far I've been re-"printing to PDF" the individual PDF files and inserting those, and it seems to insert fine - but I have almost a hundred files to insert, and I'd rather not have to do that individually for each file. Any suggestions would be greatly appreciated.

Thanks!