How do I merge Pages documents
I am on Pages 10.1 and no longer have access to anything that still has v8.1.  This means the precise instructions might be a little different, but Pages 8.1 does support these features (even if the menus or wording might be a little different).
Pages has a two major modes of operation.

Word Processing mode:  This is the default mode for most documents.  In this mode, document text is added directly to the page. If you reach the end of a page but continue typing, a new pages is added and you continue to type on the new page.  If you delete some of the text from the first page, all text is moved up (including text from adjacent pages).
Page Layout mode:  This mode works like a publishing system and is easier to use if you are creating newspapers, newsletters, or magazine style documents... where you have multiple articles and an article might start on one page, but finish on a later page -- even with other articles interspersed between them.  In this mode, you don't type directly onto a page.  Instead you create a text frame and place it on the page and type only within the frame.  If you run out of space, the frame will not automatically grow nor will new pages automatically get created.  You manually added pages as needed and add more text frames as needed... but you link text frames together if they all belong to the same article in the publication.  When a text frame is full, text automatically flows into and continues in the next text frame (provided you linked it to the same article).

You do a little more setup to use Page Layout mode because you have to manually create and size your text frames where you want them ... but the advantage is that things go exactly where you want them.
Also, in Page Layout mode, each page is a discrete thing.  If you move a page, the whole page and everything on it moves along with it and everything is exactly as you placed.  This is true wether you are moving it to a new location within the same document, or copying it to a completely different document (assuming the destination document is also using Page Layout mode.)
Convert to Page Layout Mode
Before you do this, beware the danger... in Page Layout mode, you cannot apply text directly onto a page.  E.g. if you create a new blank document, switch it to Page Layout mode, and then try to type... you'll notice it won't let you.  You must drop a text box onto the page and then you can type inside the text box.  It's probably worth doing some experiments with this to understand how it works before attempting to convert a document you care about (make a backup before you make any changes).
When you ultimately do perform a conversion from Word Processing mode to Page Layout mode, any text you had typed directly onto a page will disappear.  It will not automatically create text boxes for you and move your text into those boxes.  This is something you have to do manually.
There are two ways to perform the conversion (but I'm using Pages 10.1 and cannot promise this looked the same in Pages 8.1):
Using the 'File' Menu to Convert
Navigate to the 'File' menu and you should notice a menu choice that will either say &quot;Convert to Page Layout&quot; or &quot;Convert to Word Processing&quot;.  If you see &quot;Convert to Page Layout&quot; it means you are currently in Word Processing mode.  If you see &quot;Convert to Word Processing&quot; it means you are in Page Layout mode.
Using the Document Inspector Panel
The other method is to use the Document inspector.  I've added an image for clarity.

To use this method,

Select the 'Document' inspector panel icon on the tool-bar.
Select the 'Document' tab within the Document inspector panel.
Check to see if 'Document Body' is enabled.

If Document Body is enabled, you are in Word Processing mode (and can type directly on the page).  If Document Body is not enabled, you are in Page Layout mode and can only type text within a text-frame.
If you attempt to change the mode you will be asked to confirm that you really want to change.  Remember... any text that was directly on a page (not within a text frame) will be lost.  Make sure you manually copy this text, create text boxes, and then paste your text into the text box(es).
Linking Text Boxes
When you create a new text box, if you select the box you'll notice the control points located not the sides and corners, and a round empty circle in the top-center of the box.  Click that circle and it will turn into a number '1' and get a color assignment.  This means it is the 1st text box associated with your thread (a thread represents an article).
Create a new text box and repeat by clicking the circle.  It will automatically fill in with a '2' (indicating it is the 2nd text frame in the thread.)  But notice if you hover-over the '2', a tiny arrow appears.   This means this text box is now &quot;linked&quot; to the first text box.  As you type and fill the box, text will autoflow into the next box that is linked to the same thread.  If you click on the colored number in the top-center of any text box you get a sub-menu and can choose to link that box to a different thread/article or create a new thread, or just unlink the box (remove from thread).
Copy &amp; Paste
In Page Layout mode (and only in Page Layout mode) you can move entire pages &quot;as is&quot; -- the page will look exactly the same in the new destination.  You cannot do this in Word Processing mode.

Select &quot;View&quot; -&gt; &quot;Page Thumbnails&quot;
Select the individual page or pages that you want to copy (it is only possible to select individual pages if you are in Page Layout mode).
Select &quot;Edit&quot; -&gt; &quot;Copy&quot;
In the destination document, select &quot;Edit&quot; -&gt; &quot;Paste&quot;

The pages will come across as they were.  This is because in this mode, pages simply hold &quot;objects&quot; -- objects are things like text boxes, images, charts or graphs, etc. etc.  Each page knows which objects are located on that page and the relative positioning or layout of each object.  Hence the name &quot;Page Layout&quot; mode.
Nuances of Either Mode
You can insert objects inline within text in either mode.  E.g. insert a picture in the middle of an article in Word Processing mode or in Page Layout mode and that image placement is relative to the point in your article ... not to a spot on the page.  In other words, add or delete some text in the article located above the image and you'll see the image moves up or down.
But you can also create an object on a page (such as an image) where it's location is pinned to the page itself and not tied to a location within an article (it wont move as you edit the article... the article will move and flow around the other object -- depending on the rules you set.
If you copy a range of an text in an article and that range includes an &quot;inline&quot; image, the image will get copied along with the text.  If you copy a range of text where the image is it's own object (not inline), then you'll only get the text and the other objects are left behind.  You have to manually copy those separate objects in a separate operation.