Well, not entirely. The PageRank algorithm is very cleverly balanced. Just like the conservation of energy in physics with every reaction, PageRank is also conserved with every calculation. For instance, if a page with a starting PageRank of 4 has two outgoing links on it, we know that the amount of PageRank it passes on is divided equally between all of its outgoing links. In this case 4 / 2 = 2 units of PageRank is passed on to each of 2 separate pages, and 2 + 2 = 4 - so the total PageRank is preserved!
Note: There are scenarios where you may find that total PageRank is not conserved after a calculation. PageRank itself is supposed to represent a probability distribution, with the individual PageRank of a page representing the likelihood of a 'random surfer' chancing upon it. I have discussed this concept in more detail here).
On a much larger scale, supposing Google's index contains a billion pages, each with a PageRank of 1, the total PageRank across all pages is equal to a billion. Moreover, each time we recalculate PageRank, no matter what changes in PageRank may occur between individual pages, the total PageRank across all one billion pages will still add up to a billion.
Firstly, this means that although we may not be able to change the total PageRank across all pages, by strategic linking of pages within our site, we can affect the distribution of PageRank between pages. For instance, we may want most of our visitors to come into the site through our home page. We would therefore want our home page to have a higher PageRank relative to other pages within the site. We should also recall that all of the PageRank of a page is passed on and divided equally between each of the outgoing links on a page. We would therefore want to keep as much combined PageRank as possible within our own site without passing it on to external sites and losing its benefit. This means we would want any page with lots of external links (ie. links to other people's web sites) to have a lower PageRank relative to other pages within the site to minimise the amount of PageRank which is 'leaked' to external sites. Bear in mind also our earlier statement, that PageRank is simply a multiplying factor applied once Google's other calculations regarding relevance have already been calculated. We would therefore want our more keyword-rich pages to also have a higher relative PageRank.
Secondly, if we assume that every new page in Google's index begins its life with a PageRank of 1, there is a way we can increase the combined PageRank of pages within our site - by increasing the number of pages! A site with 10 pages will start life with a combined PageRank of 10 which is then redistributed through its hyperlinks. A site with 12 pages will therefore start with a combined PageRank of 12. We can thus improve the PageRank of our site as a whole by creating new content (ie. more pages) and then control the distribution of that combined PageRank through strategic interlinking between the pages.
And this is the purpose of the PageRank Calculator - to create a model of the site on a small scale including the links between pages, and see what effect the model has on the distribution of PageRank.