The reality is that nothing is secure.
Hackers can get into just about anything they want.
While the Pay Pal site is probably as secure as physically possible, the weakness would be your own computer.
If you have Norton or one of the other antivirus programs running , you can stop the novice hacks all day long , but the sophisticated thieves would laugh at it.
I don't do banking or anything that requires my social security number or routing numbers online unless it is impossible to do it any other way.
Then I don't put it in on my computer, I call the place I am dealing with and have them enter it from their site.
That still leaves an opening either by the person at the other end , or if their site is hacked.
But, it is a lot less likely they are more vulnerable than me.