Turnitin has a limit of
40Mb 100Mb (currently) for files,
some email apps get upset if you try and attach more than 20Mb,
many agencies insist on files under 5Mb.
If your pdf is too big you can easily reduce the size using some simple tools.
On a mac open the pdf in Preview and pick Export (Not export as PDF)
Use a different filename so you don’t lose your original file.
Select “PDF” as the format, and “Reduce File Size” from the Quartz Filters.