Approach to combine pairs of landscape pdf pages into A4 ones, using Deno and pdf-lib. Page count will be at most half + 1.

.ts

import {
  PageSizes,
  PDFDocument,
  PDFEmbeddedPage,
} from "https://cdn.skypack.dev/pdf-lib@1.17.1?dts";

interface FitPage {
  page: PDFEmbeddedPage;
  new_dims: {
    width: number;
    height: number;
  };
}

const pdf_out = await PDFDocument.create();
const pages = await pdf_out.embedPages(
  (await PDFDocument.load(await Deno.readFile(Deno.args[0]))).getPages(),
);
const [width, height] = PageSizes.A4;

let currentPage = 0;
while (currentPage <= pages.length - 1) {
  const out_page = pdf_out.addPage(PageSizes.A4);

  const fit_pages: Array<FitPage> = [];
  for (let i = 0; i < 2; i++) {
    const page = pages[currentPage + i];
    if (page) {
      const size = page.size();
      const scaling_ratio = Math.min(
        width / size.width,
        height * .5 / size.height,
      );
      const new_dims = page.scale(scaling_ratio);

      fit_pages.push({ page, new_dims });
    }
  }
  currentPage += 2;

  fit_pages.forEach(({ page, new_dims }, idx) => {
    out_page.drawPage(page, {
      x: (width - new_dims.width) / 2,
      y: (idx % 2 ? 0 : height / 2) + (height / 2 - new_dims.height) / 2,
      width: new_dims.width,
      height: new_dims.height,
    });
  });
}

Deno.writeFile(`new_${Deno.args[0]}.pdf`, await pdf_out.save());

deno run --allow-read --allow-write .ts input.pdf