This article explains how to enable or disable the built-in PDF reader on Google Chrome using a computer, but also how to change the default PDF reader on Windows or macOS.
Steps
Method 1 of 4: Enable Chrome's PDF Reader

Step 1. Open Google Chrome on your computer
It is located in the folder Applications of macOS and in the section All programs from the Windows "Start" menu.

Step 2. Click on ⁝
It's a button located at the top right of the Chrome screen.

Step 3. Click on Settings

Step 4. Scroll down and click on Advanced
It is located at the bottom of the screen.

Step 5. Scroll down and click on Content Settings
It is located in the "Privacy and Security" section, almost at the bottom of the list.

Step 6. Scroll down and click on PDF Documents
It's almost at the bottom of the list.

Step 7. Click on the button
to deactivate it.
As long as the button is gray it means it's grayed out, so Chrome will automatically show the contents of PDF files instead of downloading them to your computer.
Method 2 of 4: Disable Chrome's PDF Reader

Step 1. Open Google Chrome on your computer
It is located in the folder Applications of macOS or in the section All programs from the Windows "Start" menu.

Step 2. Click on ⁝
It is located in the upper right corner of the Chrome screen.

Step 3. Click on Settings

Step 4. Scroll down and click on Advanced
It's at the bottom of the page.

Step 5. Scroll down and click on Content Settings
It is located in the "Privacy and Security" section, towards the end of the list.

Step 6. Scroll down and click on PDF Documents
It is found towards the end of the list.

Step 7. Click on the button
to activate it.
As long as the button is activated it will be colored blue, so Chrome will suggest you download the PDF files instead of showing them automatically.
Method 3 of 4: Change the Default PDF Reader on Windows

Step 1. Click on the menu
It is located in the bottom left corner of the screen.

Step 2. Click on
It is located on the left side of the menu.

Step 3. Click on App

Step 4. Click on Default Apps
It is an option found in the column on the left.

Step 5. Scroll down and click Choose default apps by file type
On the left side of the window a list will appear with various file types, each of which will be associated with a program.

Step 6. Scroll down to ".pdf"
The current default reader will appear next to this item.

Step 7. Click on the default PDF reader
For example, if it is currently set to Google Chrome, click on the browser icon. A list of programs that can read PDF files will appear.

Step 8. Click on the program you want to use
It will thus become the default program that will allow you to open PDF files on your PC.
Method 4 of 4: Change the Default PDF Reader on macOS

Step 1. While clicking on a PDF file, press Control
This will open the menu.

Step 2. Click Get Info
The current default PDF reader will appear on the screen next to "Open with".
For example, if "Preview" is the default program used to open PDF files, you will see this application

Step 3. Click on the drop-down menu next to "Open with"
A list of possible alternatives will appear.

Step 4. Click on the program you prefer
If you want to use Chrome, select "Google Chrome". Alternatively, choose another option.

Step 5. Click Edit All
A confirmation message will appear.

Step 6. Click Continue
From now on, the selected program will have the function of opening all PDF files on the Mac.