However, this default setting makes it difficult to open multiple PDF files in separate windows. You can change this setting within Acrobat and force PDF documents to open in new windows.

Learn how to always open PDF files in Protected View in Word. Stop Word from opening PDF files in Protected View using GPEDIT or REGEDIT.

Open Acrobat and click "File" in the top menu to open a popup menu. Hover the mouse pointer over "Create" and select "Combine Files Into A Single PDF." The Combine Files window appears on the screen.

Gimp is normally used to create or manipulate images files, but can also work with PDFs if you understand how it works and what it's limits are.