when tap and hold on the Print button (of App) at the top right corner doesn’t show Save option, how to Print as PDF?

You can print almost anything to PDF… but the process takes a few steps.

Start by using the Print option in the App from which you are attempting to print; depending upon the App, you may need to use the Share option to find Print.

When the familiar iPad Print page is open, select any print pagination or scaling option as required - then you use a two-finger zoom gesture on any of the thumbnail images that you see to the left of the print page dialogue:

This gesture will open a PDF view of the page. From this PDF page tap the Share icon at top-right:

Tapping the Share icon opens the familiar Share sheet. Scroll down the page until you find the Save to Files option:

Tap Save to Files - and choose the destination path to which you wish to save your PDF. When you have selected the path, finish by tapping the Save button.

From the Share sheet, you can obviously choose any of the other available actions - should you choose to do so.

While the described method to create a PDF doesn’t require a physical printer to be available, it does use the native (i.e., built-in) Print features of iPadOS.

Presumably the App for which you have included a screenshot doesn’t have a print option - or doesn’t have a print option that uses the native functions of iPadOS. As I don’t read chinese, I cannot decipher the screenshot.