Yes OCR can be used to reduce the file size of a PDF as pdf is image based and OCR converts that image into text which reduces the size of the file. This may make great change to the file size and it can reduce the overall burden of data.
But if this reduces the impact on files, it can also result in slower downloads for users, particularly if they have many PDFs. The biggest challenge is that there is a lot of variation in the sizes of PDF files with some sizes being 100% the same, other PDF's having much smaller and other PDFs have large file sizes. When scanning your data for OCR it is important to compare sizes of the PDF files before you launch OCR, so you can optimize the file size of the PDF file. Some OCR applications offer to export the PDF to a text file for comparison, but it is better to run a text to PDF conversion from the PDF using one of the various free OCR tools so that you can compare your files then. This will allow you to determine that the OCR has shrunk the PDF's from 100% the same size to.