The maximum file size allowed is 4Mb. You will need to reduce the PDF file size or use a Compressed (Zipped) Folder.  


To Zip a File:

  1. Locate the file or folder that you want to zip.
  2. Press and hold (or right-click) the file or folder, select (or point to) Send to, and then select Compressed (zipped) folder. 
  3. A new zipped folder with the same name is created in the same location.


To reduce a file size:

  1. Open the PDF
  2. Click File >Save as Other
  3. Select Reduce Size PDF 



Resources:

Zip File: https://www.howtogeek.com/178146/htg-explains-everything-you-need-to-know-about-zipped-files/

Compress PDF Online: https://smallpdf.com/compress-pdf


More information:

https://www.onelegal.com/blog/5-ways-to-reduce-the-size-of-a-pdf/