Interesting question. I don’t recall any info in the manual that addressed the number of pdfs you could have in a library but pg. 91 of the X3 manual states: “there is no limit to the number of references you can create in an EndNote library…[but] it is recommended that the number not exceed 100,000 to maintain efficient performance managing the database”.
It may be a leap but if it’s possible to create a library of 100,000 references, it would seem reasonable to expect being able to attach a pdf for each reference — assuming you had enough memory on the computer’s internal hard drive or an external hard drive.
In any event, 720+ pdf files doesn’t seem problematic.