Everything you download goes straight from Pinterest to your Downloads folder. The list of what was downloaded, your settings and the download history the extension keeps are stored in your browser's own extension storage and never uploaded.
By default the extension does not send your Pinterest login with its requests: it reads public profiles and boards the way a signed-out visitor would. Only if you tick "Use my Pinterest login" (needed for your own secret boards) does it send it, and then only to Pinterest.
Three profiles are free per Pinterest account, and a licence key unlocks the rest. To count them, before each profile or board download the extension tells the NovaByteWorks licence server (api.novabyteworks.com): the id and username of the Pinterest account signed in in your browser, the username of the profile being downloaded (or of the board's owner), and your licence key if you entered one. The server keeps which profiles each account has downloaded, when, and how often it reached the free limit, so the free count is right; it never receives your login, your files or anything else about your Pinterest account. Saving single pins does not contact it. When you buy, the payment is handled by Razorpay or Gumroad, and the server keeps the licence key, the plan and the email address you paid with, to issue and recover the key.
To make downloads faster and more reliable, the extension sends this server anonymous usage statistics:
These statistics never include which profiles, boards or pins you download, your Pinterest username or anything else about your Pinterest account, your files, or your browsing. There is no account, name or email address. Your IP address is used only to limit how often one address can send data and is not stored. The data is kept for 365 days and used only to find bugs and make downloads faster and safer. It is not sold or shared.
Your browser opens a short thank-you page, which tells this server that the install id was removed.