The most likely cause of a problem attaching a PDF file is the file size. Attachment may fail for any file larger than 2 MB.
There are different types of PDF, most importantly "image only PDF" and "PDF with text." An "image-only type PDF" is a much larger file. A "PDF with text" is usually created by programs on the computer (not scanned) to be read by other programs. You can tell which type of PDF by whether text can be selected when open in Adobe Reader.
If you scan documents as PDF, you must be careful about the image-only type. The software you use to scan, whether it's associated with your printer, scanner, or is Adobe software, should provide some information about optimization of the file size and various image settings, resolution etc. There is also software that can perform optical character recognition of scanned documents, either during scanning or after, and this can convert image only PDFs to image with text.
If you're creating the PDF from any program on the computer, there may be different options to save, print, or create the PDF, and these may produce different file sizes. PDFs can be created differently by different software, including the PDF writers that act like printers and are used by other software. You can consult help or resources for any PDF software for information on optimization or for trying a different method of creating the PDF. You can also use a different method with different software.
For example, if you have a program that has a "save as PDF" command, try instead printing to PDF with a PDF writer. Adobe and other software often automatically install the Adobe PDF writer on your computer. You can find this in the printer selection of any print dialog box from the print command of any program on your computer. There are also other PDF writers available on the Internet, including free ones like CutePDF.
Also to consider for reducing PDF size: