Combine Open Files:

1. With the target document opened, Select File > Create > Combine files into a Single PDF.
2. A Combine files window opens.
3. From the Combine files window, select Add open files. Open PDF Files dialog box opens. 
4. From the Open PDF Files dialog box, select the files that you want to combine. To select the files on macOS, press command and select the files, or for Windows, press Ctrl and select files (Windows). 
5. Select Add Files.
6. The files that you added are shown in the Combine files window. You can re-arrange the documents in the order you like. Then select Combine from the top-right.
7. The combined files open in a new tab in Acrobat. 

Combine Files:

1. Select File > Create > Combine files into a Single PDF.
2. A Combine file Window opens. 
3. From the Combine files window, select Add files. A browser window opens. You can select the files you want to combine. To select the files on macOS, press command and select the files, or for Windows, press Ctrl and select files (Windows). Select Open.
4. The files that you added are shown in the Combine files window. You can re-arrange the documents in the order you like. Then select Combine from the top-right. 
5. The combined files open in a new tab in Acrobat. 

Insert one PDF into another

1. Open the PDF you want to use as the foundation for the combined file.
2. From the All tools menu, select Organize Pages. The Organize Pages toolset is displayed.
3. From the Organize pages toolbar, select Insert > From File. (Alternatively, you can right-click a page and select Insert Pages to get the insert options.)
4. Select the PDF you want to insert and choose Open.
5. In the Insert Pages dialog box, specify where to insert the document. Select the option as per your need from the Location drop-down menu, and then select OK.
6. To leave the original PDF intact as a separate file, go to File in the upper left, select Save As, and enter a new name for the merged PDF.