I modified some details on a pdf in photoshop but after saving it again as a pdf , photoshop changes the file size somehow, even though the editing was done at only a small portion inside the page.Original file size was 839 x 595 but once saved again, it becomes 814 × 546 points. What can I do so that photoshop preserves the file size ? Thanks
Once you opened the PDF in Photoshop you rasterized everything. That is to say, you converted all the information into a pixelated version of itself. Rather than keeping all the Postscript, Font and Vector information. This expands the file size greatly.
If you need to do an edit to an existing PDF try using the Edit features in Adobe Acrobat.
To edit text and photos in Acrobat: EDIT > Edit Text & Images
Then use the edit tools to the right.
You may run into font issues if you do not have the original fonts, and some things may jump around on the pages. There are many tools out there to help you in editing PDF's (See PitStop Pro). So be sure to review all pages after editing.