Make Visualforce Pages Visible to Users

In Salesforce Classic

For "Enhanced Profile User Interface"

1. Go to Setup | Administer | Manage Users | Profiles
2. Click the Profile Name you want to edit.
3. Under Apps section, click Visualforce Page Access and click Edit.
4. Add the desired Visualforce pages to 'Enabled Visualforce Pages'.
5. Click Save.
 

For "Non-Enhanced Profile User Interface"

1. Go to Setup | Administer | Manage Users | Profiles
2. Click the Profile Name.
3. Scroll to the bottom and click edit for "Enabled Visualforce Page Access". 
4. Add the Visualforce pages 
5. Click Save.

In Lightning Experience

1. Go to Setup | Administration | Users | Profiles
2. Click the Profile Name.
3. Scroll down to 'Enabled Visualforce Page Access' related list and click Edit.
4. Add the Visualforce pages.
5. Click Save.