Zooming size of pdf files and bookmarks

No, it does not change back to 'default', whenever the bookmarks are clicked. It changes back to 'Fit page to window'.

Why is this?

And it does so, because each of the bookmarks is defined with an additional parameter to do exactly this. In your PDF's code this is equivalent to this snippet:

 7319 0 obj 
    <</D [1457 0 R /Fit] /S /GoTo>>
 endobj

It's the /Fit part of the snippet where the /GoTo action tells the viewer to render the destination page (specified by /D) which the file holds in object 1457 0 obj as 'Fit to window'...

How can you change this?

Not without any modification to the PDF file itself. You cannot setup your viewer to ignore these instructions which are in the PDF code (at least I'm not aware of any viewer that does allow this...).

So you have to edit your bookmarks in your PDF file.

With Adobe Acrobat Pro you can do this by right-clicking on a bookmark, selecting Properties... from the popped-up context menu and changing the 'zoom' for that bookmark from 'Fit page to window' to '$whateveryouprefer'.

There may be other PDF editors (Foxit Reader is a good guess) which also allow bookmark editing, but I'm not familiar enough with these to tell for sure without looking it up....

Update:

If you dare, you can open your PDF in a text or hex editor. Search for all spots which contain /Fit. Change these to /XYZ (Attention, there are more than 600 of these in your file -- if you are familiar with sed, the stream editor, you could use that tool as well). That will cause Acrobat Reader to keep the current zoom factor when jumping to the target after clicking a bookmark.

open the document, go to thumbnail view, select all the pages, right click for page properties, choose action tab click add button, choose view>zoom>fit to height, close

go to bookmark view. click on bookmarks and page view is default for all