Colleague Has Problems Reading PDFs

Hello. I work for a nonprofit in Washington, DC. I live in California. When I submit expense reports (a frequent occurence) my colleague in DC ALWAYS has an issue. Sometimes she can't see/read the first page. Sometimes she can't open the document and receives an error message. Sometimes (today), she can see the first few pages but the remaining 22 (it was a 26-page PDF) are just little white squares that won't expand or be readable when clicked on.

Here are the particulars: I'm on a MacBook Pro (less than a year old) running the latest version of Catalina. She's on a Windows 10 desktop and I believe uses Acrobat Reader to open the reports.

Here's how I generate the PDF: first, I scan all my receipts (on an Epson AIO) and save as a PDF. Then I print my expense report, which is created in Excel for Mac, sign it, and scan it as a PDF. Next I open that file (with Preview) and insert the receipts file, save it and then send it.

That's pretty much it. Any thoughts on how to fix this issue would be most appreciated.

Thanks!