Click the blue, face-like icon in your Mac's Dock. The Finder window will open. Click the folder in which your PDF files are saved on the left side of the Finder window. Double-click one of the PDF files to open it in Preview, which is your Mac's default PDF-viewing application. You can also click the PDF file, click File, select Open With, and click Preview. It's a menu item in the upper-left side of your Mac's screen. A drop-down menu will appear. This option is in the View drop-down menu. You should see a window pop out of the left side of the Preview window. Hold down ⌘ Command while you click each PDF that you want to select. Click and drag the PDFs into the thumbnail bar on the left side of the Preview window. It's a menu item in the top-left corner of your Mac's screen. This option is at the bottom of the File drop-down menu. A window will open. It's in the bottom-left corner of the window. A drop-down menu will appear. You'll see this at the top of the drop-down menu. In the "Save As" text field at the top of the window, select the text and replace it with whatever you want to name your file. It's in the bottom-right corner of the window. Doing so will combine your selected PDFs into one document and save them into the same folder in which the rest of the PDFs are.

TL;DR in Indonesian:
Buka Finder. Buka lokasi penyimpanan semua berkas PDF yang ingin digabungkan. Buka salah satu berkas PDF. Klik View. Klik Thumbnails. Pilih semua berkas PDF yang lain. Tambahkan semua berkas yang sudah dipilih ke Preview. Klik File. Klik Print…. Klik kotak drop-down PDF. Klik Save as PDF…. Masukkan nama berkas. Klik Save.