How to Stop InDesign from Reducing File Size When Exporting to PDF

Just to clarify —

• There is no particular relationship between an InDesign file size and an exported PDF's size. It is not at all uncommon for a multi-megabyte ID file to produce "perfect" 100k PDFs. (For one thing, ID files are prone to file bloat, so a lot of those megs could be just junk.)
• You have extensive control over the PDF's resolution, color depth/model and other technical aspects in the export menu. It'w worth learning at least the basics of what all these standards and settings do.
• It's usual to let the PDF export set resolution and so forth for all elements, to make everything consistent and fully 'processed' according to the export settings. This is especially true for print — printers are unamused by 2MB author photos in one color model, 10MB logos in another and the like and doing such a raw, passthrough export means all further processing is in someone else's hands. It's best, 9 times out of 10, to set your own export values and let ID/Acrobat process everything to a consistent (and often much more compact, without losing any resolution or quality) result.

Hi @jayjay_2398 , in the Export>Compession tab turn off Downsampling and Compression for both RGB and Grayscale images

Hi is there a way to stop indesign reducing file size when exporting to pdf?

By @jayjay_2398

Do you mean in kB / MB or page size? 

You can change the image resolution and the compression method--this depends on your PDF export settings. Is the problem that the PDFs are not good enough quality?