Read the file limit beside the upload field and check whether it applies to each attachment or the entire submission. Keep your original, especially if it contains a digital signature. Exporting a smaller copy can change or invalidate a signature.
Prepare the file
For a PDF you created in a word processor, export again using the smaller-file option and reduce large embedded photos in the source document. For a scanned PDF, rescan at a lower resolution or in grayscale if the recipient permits it. Keep color when it carries information, such as a stamp or a highlighted field.
Check the finished copy
Open the smaller PDF, check the page count and zoom in on the smallest text. Confirm the actual file size in Properties or Get Info. If the file is still too large, reduce the largest scans first. Combine or split documents only when the portal instructions allow it.