Filings on PACER are limited by the file size of PDFs, and sometimes by the number of pages. The tricky thing is the limit on the number of bytes for a single upload differs between the federal district, bankruptcy, and circuit courts. If you've got a single PDF which is more than 10.1 MBs that you're filing with the United States District Court for the Southern District of New York, you'll need to divide it into separate PDFs.

The Administrative Office of the United States Courts provides an index listing the maximum file size for each court, entitled, "CM/ECF PDF FILE SIZE LIMITS". The allowable limits range from a minuscule 3 MB for the United States Bankruptcy Court for the Northern District of California and the Northern Mariana Islands, to 200 MB for the Bankruptcy Courts for the Western District of Pennsylvania; the Eastern District of Texas; and the Eastern District of Wisconsin. A limit of an astounding 39933288 KB (39 GB) is listed for the Third Circuit, but this is an error. The actual limit is 50 MB. The median file size limit for all federal courts is 30.5 MB.