If you want to encrypt a PDF on a Mac, the easiest method is to use the Preview app. You can use Preview to add a password to your most sensitive PDF documents to keep them secure.
如果要在Mac上加密PDF，最簡單的方法是使用“預覽”應用。 您可以使用“預覽”向最敏感的PDF文檔添加密碼，以確定它們的安全。
Preview is available as a core macOS app, so you’ll find it on any Mac computer. There’s no additional software required—all you need is a PDF document to encrypt and a Mac to encrypt it with.
預覽是macOS的核心應用程式，是以您可以在任何Mac計算機上找到它。 不需要其他軟體，您隻需要一個PDF文檔即可加密，而Mac則可以使用它進行加密。
If you’d prefer to use other software, you can password-protect PDFs using Microsoft Office instead, but Preview offers the quickest way to do it using a core Mac app.
如果您想使用其他軟體，則可以使用Microsoft Office來對PDF進行密碼保護，但是預覽版提供了使用核心Mac應用程式最快的方法。
To add a password to a PDF document on macOS, open the Preview app from the Launchpad, which you can launch from the Dock at the bottom of your Mac’s display.
要将密碼添加到macOS上的PDF文檔中，請從啟動闆打開“預覽”應用程式，您可以從Mac顯示屏底部的Dock中啟動該應用程式。
Alternatively, you can launch Preview from the Applications folder in the Finder app—you’ll also find this as an icon on your Dock.
或者，您可以從Finder應用程式的Applications檔案夾中啟動Preview-您還将在Dock上找到它作為圖示。
Once Preview is open, click File > Open from the menu bar to open the PDF document you want to encrypt with a password.
打開“預覽”後，從菜單欄中單擊“檔案”>“打開”以打開要使用密碼加密的PDF文檔。
To begin encrypting the PDF document you’ve opened in Preview, click File > Export from the menu bar.
要開始加密在預覽中打開的PDF文檔，請從菜單欄中單擊檔案>導出。
This will bring up various options to export your PDF in different formats, including the option to add a password.
這将顯示各種選項以不同的格式導出PDF，包括添加密碼的選項。
If you don’t want to overwrite your existing document, you’ll need to give your new document a name in the “Export As” box. Confirm the save location under the “Where” drop-down menu.
如果您不想覆寫現有文檔，則需要在“導出為”框中為新文檔命名。 在“位置”下拉菜單中确認儲存位置。
To add a password, click the “Encrypt” checkbox in the options menu and then type a secure password in the “Password” and “Verify” boxes immediately underneath.
要添加密碼，請單擊選項菜單中的“加密”複選框，然後在下面的“密碼”和“驗證”框中鍵入安全密碼。
Press the “Save” button to save your encrypted PDF document once you’ve confirmed your settings.
确認設定後，按“儲存”按鈕儲存加密的PDF文檔。
Your PDF document will then be exported using the options you’ve selected. Loading the document in Preview after the file has been encrypted will require you to insert a password.
然後，将使用您選擇的選項導出PDF文檔。 加密檔案後，在預覽中加載文檔将需要您輸入密碼。
Type in the document password in the “Password” box and then press Enter on your keyboard.
在“密碼”框中鍵入文檔密碼，然後按鍵盤上的Enter。
If the password is correct, this will decrypt the file and open the document. You’ll be able to continue viewing the file until you close it—once closed, you’ll need to re-enter the password to view the file again.
如果密碼正确，這将解密檔案并打開文檔。 在關閉檔案之前，您将可以繼續檢視它-一旦關閉，則需要重新輸入密碼才能再次檢視檔案。
翻譯自: https://www.howtogeek.com/660562/how-to-encrypt-a-pdf-on-a-mac/