From the comments it appears that your question is essentially this: Why is paper documentation of a transaction treated differently than electronic replies or documentation from a transaction, at least in terms of those documents containing full, unencrypted PANs? (Meaning full, unmasked credit and debit card numbers.)
Well, my first response is simply this: ideally merchants shouldn't keep materials with full PANs on/in them, even in paper form . Although the PCI Requirements that apply to SAQ-A merchants do not prohibit having cleartext card numbers visible on paper documentation, they do impose requirements that you need to pay attention to in terms or the creation, storage, handling, use, and destruction of such paper documents. For more complete information on those requirements see "Can we print cardholder data under the PCI DSS Compliance framework and stay compliant?" But the bottom line is that creating, storing, or even just receiving printed materials with full PAN data in them at best creates more compliance and security headaches for you, and at worst, of course, increases the risk that card data will be compromised. Thus, unless you really, really have a special, unavoidable need to receive and temporarily store card numbers on paper from your processor (and frankly my mind is blanking on thinking up any scenario where that would be true) the best way forward is simply not to receive or store them at all. And frankly if you're dealing with a payment processor who is sending you paper material with full PANs in it without an exceptionally good reason I would firmly advise that you to take another good look at whether you want to stick with that processor.
Secondly, if we're pondering the question of why you might be allowed to store clear-text card numbers on paper at all when you are forbidden from doing so electronically, well the answer to that is straight-forward enough: the Internet is a dangerous place, and any machine connected to it is exposed to that danger. Any machine or network that psyically has an electronic connection is exposed, in (in theory) to electronic attack from any of the more that two billion people who now have some kind of access to it, from every corner of the world. Tireless automated scanning tools or human attackers checking the security of every merchant that they're aware of the existence of can check the router that ties your small network to the Internet for security configuration problems. Problems that can allow and attacker to access everything on your network as if he or she was physically in your building/s and plugging in with an Ethernet cable or connecting to your wifi. Automated phishing campaigns bombard hundreds of thousands or millions of users with emails containing links that lead to websites serving malware or come with attached files that are infected with it. In either case, an attacker again gains access to your internal network or, worse, is able to get a copy of any card numbers that are entered in to an infected machine or even any card numbers that ever come into its memory, even for a few seconds.
By contrast, paper documents are simply not as exposed to bad guys to the same degree and in the same way. Only people who know that you keep the data and where can deliberately try to steal it, a far different situation--though still a concerning one-- than a situation where PANs are kept as electronic information on a device that can interact with a network that two billion people. (And lots of electronic scans running on it from bad actors, hunting for weakly defended systems and improperly-secured financial data on them.) That being said, as discussed above the better practice, by far, is not to store any unencrypted card numbers anywhere in your operations, whether in electronic or paper form.
