Short answer - you can't do what you are trying to do: the different renderers handle pagination differently, but appropriately for their output.
The HTML renderer is optimised for screen-based reading and generally allows more content per page than the print renderer does as the print renderer is constrained by the paper size that it formats to. Thus the HTML renderer allows more content on fewer pages for a better browser experience whereas the print and PDF renderers have to conform strictly to the page length.
The best illustration of this is the Excel renderer - the Excel renderer renders the entire report onto a single worksheet in most cases (for reports with grouping and page breaks set on the group footer it will render each group on its own worksheet). You wouldn't want the Excel renderer to artificially create worksheets to try to "paginate" your report or to put it all in one worksheet but insert the header into the spreadsheet rows every "page". It does the appropriate thing which is to include all the data in one big worksheet even though that may be logically thought of as one big "page".
The HTML renderer page length is determined (more accurately, influenced) by the InteractiveHeight attribute of the report (in the InteractiveSize property in the Properties pane for the report). However, the interactive height is an approximation rather than a fixed page break setting and your page breaks may still not conform to the print version even if you set InteractiveHeight to the same length as your target page length. This is because the HTML renderer will vary the page length to group the data together better so the interactive page breaks happen around about, but not always exactly, where the interactive height is set.
This is what is happening in your scenario where the report viewer shows 50 records on one page but the PDF has 45 on the first page and 5 on the second page. The report viewer is making the decision that since there are only a few records left to display it will just include them all on the one page rather than force the user to scroll even though the interactive height will be exceeded. Thus you get a better user experience but a variance in pages between renderers. The important thing about the report is the data and the experience with working with that data in that renderer, not that the pages are the same length no matter how you look at it.
See this discussion of rendering behaviour for more information on why what you are trying to achieve isn't achievable. Just educate your users that the browser pagination is optimised for their viewing pleasure.
