~~The problem with the tikzposter class is, that amongst some other questionable techniques (like nesting tikzpictures), it also wraps the whole document into a `center` environment which makes it nearly impossible to add page breaks. However you can trick the class by placing all additional pages within `\AtEndDocument{...}` which will only be executed once the class already closed its `centre` environment~~ (many thanks to @Ulrike Fischer for pointing out this was nonsense) 

The problem with the tikzposter class is, that amongst some other questionable techniques (like nesting tikzpictures), it hides the surrounding `tikzpicture` environment from the user in `\AtBeginDocument{}` and `\AtEndDocument{}`. The `tikzpicture` can not be broken across pages, but you can manually close and reopen it like this:

```
\documentclass{tikzposter} 
\title{Title}
\institute{Inst} 
\author{Auth}
\titlegraphic{Logo}
\usetheme{Basic} 

\AtEndDocument{%

  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  % 
  % page 2
  % 
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \pagebreak
  \title{another title}
  \begin{center}
  \begin{tikzpicture}
    % Background
    \coordinate (topright) at (0.5\textwidth, 0.5\textheight);
    \coordinate (bottomleft) at (-0.5\textwidth, -0.5\textheight);
    \clip (bottomleft) rectangle (topright);
    %
    % Draw background
    \begin{pgfonlayer}{backgroundlayer}
      \fill[inner sep=0pt, line width=0pt, color=backgroundcolor] (bottomleft) rectangle (topright);
    \end{pgfonlayer}
    \maketitle  
   \block{BlocktitleA}{Blocktext}  
   \begin{columns} 
      \column{0.3}  
      \block{BlocktitleB}{Blocktext}
      \column{0.7}
      \block{BlocktitleC}{Blocktext}
      \note{Notetext}  
    \end{columns}
  \end{tikzpicture}
  \end{center}
  
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  % 
  % page 3
  % 
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \pagebreak
  \title{yet another title}
  \begin{center}
  \begin{tikzpicture}
    % Background
    \coordinate (topright) at (0.5\textwidth, 0.5\textheight);
    \coordinate (bottomleft) at (-0.5\textwidth, -0.5\textheight);
    \clip (bottomleft) rectangle (topright);
    %
    % Draw background
    \begin{pgfonlayer}{backgroundlayer}
      \fill[inner sep=0pt, line width=0pt, color=backgroundcolor] (bottomleft) rectangle (topright);
    \end{pgfonlayer}
    \maketitle  
   \block{BlocktitleA}{Blocktext}  
   \begin{columns} 
      \column{0.3}  
      \block{BlocktitleB}{Blocktext}
      \column{0.7}
      \block{BlocktitleC}{Blocktext}
      \note{Notetext}  
    \end{columns}
  \end{tikzpicture}
  \end{center}  
  
}

\begin{document}
 
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  % 
  % page 1
  % 
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \maketitle  
  \block{BlocktitleA}{Blocktext}  
  \begin{columns} 
    \column{0.3}  
    \block{BlocktitleB}{Blocktext}
    \column{0.7}
    \block{BlocktitleC}{Blocktext}
    \note{Notetext}  
  \end{columns}

\end{document}
```

That said, it might be easier to

- prepare 5 separate documents and glue them together, for example with the `pdfpages` package or similar tools

- use another class. For example I normally use the `beamer` class to create posters (maybe in combination with the `beamerposter` package). This class can automatically deal with multiple pages

An alternative to `tikzposter` could be `tcbposter` (from `tcolorbox`). You decide poster size inside any document class and can combine several posters inside the same document. A poster is not breakable between pages, but text can flow inside poster blocks.

A short example based on the `tcolorbox` documentation:

```
\documentclass{article}

\usepackage{tcolorbox}
\tcbuselibrary{poster}

\begin{document}

\begin{tcbposter}
\posterbox[adjusted title=My box]{name=A,column=1,row=1,span=3,between=top and bottom}{My first box}
\end{tcbposter}

\begin{tcbposter}
\posterbox[adjusted title=Another title]{name=A,column=1,row=1,span=3,between=top and bottom}{My first box}
\end{tcbposter}

\end{document}
```