Annoying PDF file issue - trying to split page

[swissdude]

I have a PDF document sent to me - it is 57 pages

All the pages are in the same format except the front page

I just need to take the front page and change the image on it and then keep everything else as is.

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I found a tool called PDF Free Tools that lets you merge loads of PDF's into 1 and split and all sorts of things but 1 problem...

I change the image I want to change on the INTRO page

Then I go to save them all again as 1 PDF but it only makes one layout.

So the first page looks strange coz it has empty white space all around.

The other 56 pages are fine as they are the format width the rest of the pages are.

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how can I get past this obstacle?

i just want to edit 1 page which is A4 portrait size then combine and save it with all the other pages which are A4 landscape size?

so annoying

any help much appreciated